branch -m: allow renaming a yet-unborn branch

In one of the next commits, we would like to give users some advice
regarding the initial branch name, and how to modify it.

To that end, it would be good if `git branch -m <name>` worked in a
freshly initialized repository without any commits. Let's make it so.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -538,7 +538,9 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
 		strbuf_addf(&logmsg, "Branch: renamed %s to %s",
 			    oldref.buf, newref.buf);
 
-	if (!copy && rename_ref(oldref.buf, newref.buf, logmsg.buf))
+	if (!copy &&
+	    (!head || strcmp(oldname, head) || !is_null_oid(&head_oid)) &&
+	    rename_ref(oldref.buf, newref.buf, logmsg.buf))
 		die(_("Branch rename failed"));
 	if (copy && copy_existing_ref(oldref.buf, newref.buf, logmsg.buf))
 		die(_("Branch copy failed"));

t/t0001-init.sh

@@ -571,4 +571,12 @@ test_expect_success 'invalid default branch name' '
 	test_i18ngrep "invalid branch name" err
 '
 
+test_expect_success 'branch -m with the initial branch' '
+	git init rename-initial &&
+	git -C rename-initial branch -m renamed &&
+	test renamed = $(git -C rename-initial symbolic-ref --short HEAD) &&
+	git -C rename-initial branch -m renamed again &&
+	test again = $(git -C rename-initial symbolic-ref --short HEAD)
+'
+
 test_done