rerere gc: honor configuration and document it

Two configuration to control the expiration of rerere records
are introduced and documented.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/config.txt

@@ -192,6 +192,16 @@ format.headers::
 	Additional email headers to include in a patch to be submitted
 	by mail.  See gitlink:git-format-patch[1].
 
+gc.rerereresolved::
+	Records of conflicted merge you resolved earlier are
+	kept for this many days when `git rerere gc` is run.
+	The default is 60 days.  See gitlink:git-rerere[1].
+
+gc.rerereunresolved::
+	Records of conflicted merge you have not resolved are
+	kept for this many days when `git rerere gc` is run.
+	The default is 15 days.  See gitlink:git-rerere[1].
+
 gitcvs.enabled::
 	Whether the cvs pserver interface is enabled for this repository.
 	See gitlink:git-cvsserver[1].

Documentation/git-rerere.txt

@@ -7,7 +7,7 @@ git-rerere - Reuse recorded resolve
 
 SYNOPSIS
 --------
-'git-rerere' [clear|diff|status]
+'git-rerere' [clear|diff|status|gc]
 
 DESCRIPTION
 -----------
@@ -55,7 +55,11 @@ for resolutions.
 'gc'::
 
 This command is used to prune records of conflicted merge that
-occurred long time ago.
+occurred long time ago.  By default, conflicts older than 15
+days that you have not recorded their resolution, and conflicts
+older than 60 days, are pruned.  These are controlled with
+`gc.rerereunresolved` and `gc.rerereresolved` configuration
+variables.
 
 
 DISCUSSION

builtin-rerere.c

@@ -362,6 +362,17 @@ static int do_plain_rerere(struct path_list *rr, int fd)
 	return write_rr(rr, fd);
 }
 
+static int git_rerere_config(const char *var, const char *value)
+{
+	if (!strcmp(var, "gc.rerereresolved"))
+		cutoff_resolve = git_config_int(var, value);
+	else if (!strcmp(var, "gc.rerereunresolved"))
+		cutoff_noresolve = git_config_int(var, value);
+	else
+		return git_default_config(var, value);
+	return 0;
+}
+
 int cmd_rerere(int argc, const char **argv, const char *prefix)
 {
 	struct path_list merge_rr = { NULL, 0, 0, 1 };
@@ -371,6 +382,8 @@ int cmd_rerere(int argc, const char **argv, const char *prefix)
 	if (stat(git_path("rr-cache"), &st) || !S_ISDIR(st.st_mode))
 		return 0;
 
+	git_config(git_rerere_config);
+
 	merge_rr_path = xstrdup(git_path("rr-cache/MERGE_RR"));
 	fd = hold_lock_file_for_update(&write_lock, merge_rr_path, 1);
 	read_rr(&merge_rr);