[PATCH v5 2/4] branch -m: allow renaming a yet-unborn branch

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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>
---
 builtin/branch.c | 4 +++-
 t/t0001-init.sh  | 8 ++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/builtin/branch.c b/builtin/branch.c
index efb30b88206..200da319f1d 100644
--- a/builtin/branch.c
+++ b/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"));
diff --git a/t/t0001-init.sh b/t/t0001-init.sh
index 69a320489fc..bb23e56a165 100755
--- a/t/t0001-init.sh
+++ b/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
-- 
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