[PATCH 2/3] object-name: don't allow @ as a branch name

[Kristoffer Haugsbakk <code@khaugsbakk.name>]

`HEAD` is an invalid branch name.[1]  But the `@` synonym is allowed.
This is just as inconvenient since commands like `git checkout @` will,
quite sensibly, do `git checkout HEAD` instead of checking out that
branch; in turn there is no practical reason to use this as a branch
name since you cannot even check out the branch itself (only check out
the commit which `refs/heads/@` points to).

† 1: a625b092cc5 (branch: correctly reject refs/heads/{-dash,HEAD},
    2017-11-14)

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
---
 object-name.c                         | 3 ++-
 t/t3204-branch-name-interpretation.sh | 9 ++-------
 2 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/object-name.c b/object-name.c
index 42e3ba4a77a..56b288ff4c3 100644
--- a/object-name.c
+++ b/object-name.c
@@ -1763,7 +1763,8 @@ int strbuf_check_branch_ref(struct strbuf *sb, const char *name)
 	strbuf_splice(sb, 0, 0, "refs/heads/", 11);
 
 	if (*name == '-' ||
-	    !strcmp(sb->buf, "refs/heads/HEAD"))
+	    !strcmp(sb->buf, "refs/heads/HEAD") ||
+	    !strcmp(sb->buf, "refs/heads/@"))
 		return -1;
 
 	return check_refname_format(sb->buf, 0);
diff --git a/t/t3204-branch-name-interpretation.sh b/t/t3204-branch-name-interpretation.sh
index 594e3e43e12..7dcd1308f8c 100755
--- a/t/t3204-branch-name-interpretation.sh
+++ b/t/t3204-branch-name-interpretation.sh
@@ -119,13 +119,8 @@ test_expect_success 'disallow deleting remote branch via @{-1}' '
 	expect_branch refs/heads/origin/previous two
 '
 
-# The thing we are testing here is that "@" is the real branch refs/heads/@,
-# and not refs/heads/HEAD. These tests should not imply that refs/heads/@ is a
-# sane thing, but it _is_ technically allowed for now. If we disallow it, these
-# can be switched to test_must_fail.
-test_expect_success 'create branch named "@"' '
-	git branch -f @ one &&
-	expect_branch refs/heads/@ one
+test_expect_success 'disallow branch named "@"' '
+	test_must_fail git branch -f @ one
 '
 
 test_expect_success 'delete branch named "@"' '
-- 
2.46.1.641.g54e7913fcb6