checkout: avoid BUG() when hitting a broken repository

When 9081a42 (checkout: fix "branch info" memory leaks, 2021-11-16)
cleaned up existing memory leaks, we added an unrelated sanity check
to ensure that a local branch is truly local and not a symref to
elsewhere that dies with BUG() otherwise.  This was misguided in two
ways.  First of all, such a tightening did not belong to a leak-fix
patch.  And the condition it detected was *not* a bug in our program
but a problem in user data, where warning() or die() would have been
more appropriate.

As the condition is not fatal (the result of computing the local
branch name in the code that is involved in the faulty check is only
used as a textual label for the commit), let's revert the code to
the original state, i.e. strip "refs/heads/" to compute the local
branch name if possible, and otherwise leave it NULL.  The consumer
of the information in merge_working_tree() is prepared to see NULL
in there and act accordingly.

cf. https://bugzilla.redhat.com/show_bug.cgi?id=2042920

Reported-by: Petr Šplíchal <[email protected]>
Reported-by: Todd Zullinger <[email protected]>
Helped-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

builtin/checkout.c

@@ -1094,9 +1094,6 @@ static int switch_branches(const struct checkout_opts *opts,
 		const char *p;
 		if (skip_prefix(old_branch_info.path, prefix, &p))
 			old_branch_info.name = xstrdup(p);
-		else
-			BUG("should be able to skip past '%s' in '%s'!",
-			    prefix, old_branch_info.path);
 	}
 
 	if (opts->new_orphan_branch && opts->orphan_from_empty_tree) {

t/t2018-checkout-branch.sh

@@ -85,6 +85,19 @@ test_expect_success 'setup' '
 	git branch -m branch1
 '
 
+test_expect_success 'checkout a branch without refs/heads/* prefix' '
+	git clone --no-tags . repo-odd-prefix &&
+	(
+		cd repo-odd-prefix &&
+
+		origin=$(git symbolic-ref refs/remotes/origin/HEAD) &&
+		git symbolic-ref refs/heads/a-branch "$origin" &&
+
+		git checkout -f a-branch &&
+		git checkout -f a-branch
+	)
+'
+
 test_expect_success 'checkout -b to a new branch, set to HEAD' '
 	test_when_finished "
 		git checkout branch1 &&