create_branch: use xstrfmt for reflog message

We generate a reflog message that contains some fixed text
plus a branch name, and use a buffer of size PATH_MAX + 20.
This mostly works if you assume that refnames are shorter
than PATH_MAX, but:

  1. That's not necessarily true. PATH_MAX is not always the
     filesystem's limit.

  2. The "20" is not sufficiently large for the fixed text
     anyway.

Let's just switch to a heap buffer so we don't have to even
care.

Signed-off-by: Jeff King <peff@peff.net>

branch.c

@@ -296,14 +296,12 @@ void create_branch(const char *name, const char *start_name,
 	if (!dont_change_ref) {
 		struct ref_transaction *transaction;
 		struct strbuf err = STRBUF_INIT;
-		char msg[PATH_MAX + 20];
+		char *msg;
 
 		if (forcing)
-			snprintf(msg, sizeof msg, "branch: Reset to %s",
-				 start_name);
+			msg = xstrfmt("branch: Reset to %s", start_name);
 		else
-			snprintf(msg, sizeof msg, "branch: Created from %s",
-				 start_name);
+			msg = xstrfmt("branch: Created from %s", start_name);
 
 		transaction = ref_transaction_begin(&err);
 		if (!transaction ||
@@ -314,6 +312,7 @@ void create_branch(const char *name, const char *start_name,
 			die("%s", err.buf);
 		ref_transaction_free(transaction);
 		strbuf_release(&err);
+		free(msg);
 	}
 
 	if (real_ref && track)