[PATCH 03/15] t5510: document fetch with "./"-prefixed branch.<name>.merge

[Jon Simons <jon@jonsimons.org>]

refname_match() formats each candidate through mkpath(), which strips
a leading "./", so a branch.<name>.merge value of "./refs/heads/main"
matches "refs/heads/main" in both places that fetch compares them:

 - branch_merge_matches() when marking fetched refs for merge
   in FETCH_HEAD

 - find_ref_by_name_abbrev() when the merge source is not covered by the
   fetch refspec and is looked up amongst the remote advertised refs

Only a hand-edited config reaches these paths: clone, branch
--set-upstream-to, fetch --set-upstream, and push -u write only names
that have passed check_refname_format(), which rejects a leading "./".

Add two fetch tests asserting that such a "./"-prefixed value does not
match for these cases.  Both are test_expect_failure because the name
matches today:

 - The default fetch refspec should not match "./refs/heads/main"
   with fetched "refs/heads/main" and mark the ref for merge.

 - A protocol v0 fetch with refspec that omits "refs/heads/main"
   should not match "./refs/heads/main" against the remote
   advertised "refs/heads/main".

An upcoming commit stops using mkpath() in refname_match(), at which
point the tests are toggled to test_expect_success.

Signed-off-by: Jon Simons <jon@jonsimons.org>
---
 t/t5510-fetch.sh | 44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/t/t5510-fetch.sh b/t/t5510-fetch.sh
index 300bd5396d..0303784b1f 100755
--- a/t/t5510-fetch.sh
+++ b/t/t5510-fetch.sh
@@ -1074,6 +1074,50 @@ test_expect_success 'LHS of refspec follows ref disambiguation rules' '
 	)
 '
 
+test_expect_failure 'fetch with "./"-prefixed branch.<name>.merge does not mark any ref for merge' '
+	mkdir dotslash-merge-default-refspec &&
+	(
+		cd dotslash-merge-default-refspec &&
+		git init -b main server &&
+		test_commit -C server one &&
+		git -C server branch other &&
+		test_commit -C server two &&
+
+		# The bogus ./refs/heads/main should not match the remote refs/heads/main.
+		git clone server client &&
+		git -C client config branch.main.merge ./refs/heads/main &&
+		git -C client fetch &&
+		{
+			echo "$(git -C server rev-parse main)	not-for-merge" &&
+			echo "$(git -C server rev-parse other)	not-for-merge"
+		} >expect &&
+		cut -f -2 client/.git/FETCH_HEAD >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_failure 'fetch protocol v0 with "./"-prefixed branch.<name>.merge does not match any remote ref' '
+	mkdir dotslash-merge-fetch-protocol-v0 &&
+	(
+		cd dotslash-merge-fetch-protocol-v0 &&
+		git init -b main server &&
+		test_commit -C server one &&
+		git -C server branch other &&
+		test_commit -C server two &&
+
+		# Omit refs/heads/main from the fetch refspec so that the merge
+		# source is instead looked up among the refs the remote advertised.
+		git clone server client-v0 &&
+		git -C client-v0 config remote.origin.fetch \
+			+refs/heads/other:refs/remotes/origin/other &&
+		git -C client-v0 config branch.main.merge ./refs/heads/main &&
+		git -C client-v0 -c protocol.version=0 fetch &&
+		echo "$(git -C server rev-parse other)	not-for-merge" >expect &&
+		cut -f -2 client-v0/.git/FETCH_HEAD >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_expect_success 'fetch.writeCommitGraph' '
 	git clone three write &&
 	(
-- 
2.55.0