replay: add replay.defaultAction config option

Add a configuration option to control the default behavior of git replay
for updating references. This allows users who prefer the traditional
pipeline output to set it once in their config instead of passing
--update-refs=print with every command.

The config option uses enum string values for extensibility:
  * replay.defaultAction = update-refs (default): atomic ref updates
  * replay.defaultAction = show-commands: output commands for pipeline

The command-line --update-refs option always overrides the config setting,
allowing users to temporarily change behavior for a single invocation.

Implementation details:

In cmd_replay(), before parsing command-line options, we read the
configuration using repo_config_get_string_tmp(). If the config variable
is set, we validate the value and map it to an internal mode:

  Config value         Internal mode    Behavior
  ────────────────────────────────────────────────────────────────
  "update-refs"        "yes"            Atomic ref updates (default)
  "show-commands"      "print"          Pipeline output
  (not set)            "yes"            Atomic ref updates (default)
  (invalid)            error            Die with helpful message

If an invalid value is provided, we die() immediately with an error
message explaining the valid options. This catches configuration errors
early and provides clear guidance to users.

The command-line --update-refs option, when provided, overrides the
config value. This precedence allows users to set their preferred default
while still having per-invocation control:

  git config replay.defaultAction show-commands  # Set default
  git replay --update-refs=yes --onto main topic  # Override once

The config option uses different value names ('update-refs' vs
'show-commands') compared to the command-line option ('yes' vs 'print')
for semantic clarity. The config values describe what action is being
taken, while the command-line values are terse for typing convenience.

The enum string design (rather than a boolean like 'replay.updateRefs')
allows future expansion to additional modes without requiring new
configuration variables. For example, if we later add custom format
support (--update-refs=format), we can extend the config to support
'replay.defaultAction = format' without breaking existing configurations
or requiring a second config variable.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Elijah Newren <newren@gmail.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: edith007

Documentation/config/replay.adoc

@@ -0,0 +1,14 @@
+replay.defaultAction::
+	Control the default behavior of `git replay` for updating references.
+	Can be set to:
++
+--
+* `update-refs` (default): Update refs directly using an atomic transaction.
+* `show-commands`: Output update-ref commands that can be piped to
+  `git update-ref --stdin`.
+--
++
+This can be overridden with the `--update-refs` command-line option.
+Note that the command-line option uses slightly different values
+(`yes` and `print`) for brevity, but they map to the same behavior
+as the config values.

builtin/replay.c

@@ -8,6 +8,7 @@
 #include "git-compat-util.h"
 
 #include "builtin.h"
+#include "config.h"
 #include "environment.h"
 #include "hex.h"
 #include "lockfile.h"
@@ -359,9 +360,22 @@ int cmd_replay(int argc,
 	die_for_incompatible_opt2(!!advance_name_opt, "--advance",
 				  contained, "--contained");
 
-	/* Set default mode if not specified */
-	if (!update_refs_mode)
-		update_refs_mode = "yes";
+	/* Set default mode from config if not specified on command line */
+	if (!update_refs_mode) {
+		const char *config_value = NULL;
+		if (!repo_config_get_string_tmp(repo, "replay.defaultaction", &config_value)) {
+			if (!strcmp(config_value, "update-refs"))
+				update_refs_mode = "yes";
+			else if (!strcmp(config_value, "show-commands"))
+				update_refs_mode = "print";
+			else
+				die(_("invalid value for replay.defaultAction: '%s' "
+				      "(expected 'update-refs' or 'show-commands')"),
+				    config_value);
+		} else {
+			update_refs_mode = "yes";
+		}
+	}
 
 	/* Validate update-refs mode */
 	if (strcmp(update_refs_mode, "yes") && strcmp(update_refs_mode, "print"))

t/t3650-replay-basics.sh

@@ -299,7 +299,7 @@ test_expect_success 'replay atomic guarantee: all refs updated or none' '
 	# Store original states
 	START_TOPIC1=$(git rev-parse topic1) &&
 	START_TOPIC3=$(git rev-parse topic3) &&
-	test_when_finished "git branch -f topic1 $START_TOPIC1 && git branch -f topic3 $START_TOPIC3 && rm -f .git/refs/heads/topic1.lock" &&
+	test_when_finished "git branch -f topic1 $START_TOPIC1 && git branch -f topic3 $START_TOPIC3" &&
 
 	# Create a lock on topic1 to simulate a concurrent update
 	>.git/refs/heads/topic1.lock &&
@@ -308,6 +308,9 @@ test_expect_success 'replay atomic guarantee: all refs updated or none' '
 	# This should fail atomically - neither branch should be updated
 	test_must_fail git replay --contained --onto main main..topic3 2>error &&
 
+	# Remove the lock before checking refs
+	rm -f .git/refs/heads/topic1.lock &&
+
 	# Verify the transaction failed
 	grep "failed to commit ref transaction" error &&
 
@@ -354,4 +357,46 @@ test_expect_success 'replay validates --update-refs mode values' '
 	grep "invalid value for --update-refs" error
 '
 
+test_expect_success 'replay.defaultAction config option' '
+	# Store original state
+	START=$(git rev-parse topic2) &&
+	test_when_finished "git branch -f topic2 $START && git config --unset replay.defaultAction" &&
+
+	# Set config to show-commands
+	git config replay.defaultAction show-commands &&
+	git replay --onto main topic1..topic2 >output &&
+	test_line_count = 1 output &&
+	grep "^update refs/heads/topic2 " output &&
+
+	# Reset and test update-refs mode
+	git branch -f topic2 $START &&
+	git config replay.defaultAction update-refs &&
+	git replay --onto main topic1..topic2 >output &&
+	test_must_be_empty output &&
+
+	# Verify ref was updated
+	git log --format=%s topic2 >actual &&
+	test_write_lines E D M L B A >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'command-line --update-refs overrides config' '
+	# Store original state
+	START=$(git rev-parse topic2) &&
+	test_when_finished "git branch -f topic2 $START && git config --unset replay.defaultAction" &&
+
+	# Set config to update-refs but use --update-refs=print
+	git config replay.defaultAction update-refs &&
+	git replay --update-refs=print --onto main topic1..topic2 >output &&
+	test_line_count = 1 output &&
+	grep "^update refs/heads/topic2 " output
+'
+
+test_expect_success 'invalid replay.defaultAction value' '
+	test_when_finished "git config --unset replay.defaultAction" &&
+	git config replay.defaultAction invalid &&
+	test_must_fail git replay --onto main topic1..topic2 2>error &&
+	grep "invalid value for replay.defaultAction" error
+'
+
 test_done