Use "git_config_string" to simplify "builtin-gc.c" code where "prune_expire" is set

Signed-off-by: David Bryson <david@statichacks.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-gc.c

diff --git a/builtin-gc.c b/builtin-gc.c
index fac200e..7af65bb 100644
--- a/builtin-gc.c
+++ b/builtin-gc.c
@@ -26,7 +26,7 @@
 static int aggressive_window = -1;
 static int gc_auto_threshold = 6700;
 static int gc_auto_pack_limit = 50;
-static char *prune_expire = "2.weeks.ago";
+static const char *prune_expire = "2.weeks.ago";
 
 #define MAX_ADD 10
 static const char *argv_pack_refs[] = {"pack-refs", "--all", "--prune", NULL};
@@ -57,15 +57,12 @@
 		return 0;
 	}
 	if (!strcmp(var, "gc.pruneexpire")) {
-		if (!value)
-			return config_error_nonbool(var);
-		if (strcmp(value, "now")) {
+		if (value && strcmp(value, "now")) {
 			unsigned long now = approxidate("now");
 			if (approxidate(value) >= now)
 				return error("Invalid %s: '%s'", var, value);
 		}
-		prune_expire = xstrdup(value);
-		return 0;
+		return git_config_string(&prune_expire, var, value);
 	}
 	return git_default_config(var, value, cb);
 }