repack: point out a bug handling stale shallow info

A `git fetch --prune` can turn previously-reachable objects unreachable,
even commits that are in the `shallow` list. A subsequent `git repack
-ad` will then unceremoniously drop those unreachable commits, and the
`shallow` list will become stale. This means that when we try to fetch
with a larger `--depth` the next time, we may end up with:

	fatal: error in object: unshallow <commit-hash>

Reported by Alejandro Pauly.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

t/t5537-fetch-shallow.sh

@@ -186,4 +186,31 @@ EOF
 	test_cmp expect actual
 '
 
+test_expect_failure '.git/shallow is edited by repack' '
+	git init shallow-server &&
+	test_commit -C shallow-server A &&
+	test_commit -C shallow-server B &&
+	git -C shallow-server checkout -b branch &&
+	test_commit -C shallow-server C &&
+	test_commit -C shallow-server E &&
+	test_commit -C shallow-server D &&
+	d="$(git -C shallow-server rev-parse --verify D)" &&
+	git -C shallow-server checkout master &&
+
+	git clone --depth=1 --no-tags --no-single-branch \
+		"file://$PWD/shallow-server" shallow-client &&
+
+	: now remove the branch and fetch with prune &&
+	git -C shallow-server branch -D branch &&
+	git -C shallow-client fetch --prune --depth=1 \
+		origin "+refs/heads/*:refs/remotes/origin/*" &&
+	git -C shallow-client repack -adfl &&
+	test_must_fail git -C shallow-client rev-parse --verify $d^0 &&
+	! grep $d shallow-client/.git/shallow &&
+
+	git -C shallow-server branch branch-orig D^0 &&
+	git -C shallow-client fetch --prune --depth=2 \
+		origin "+refs/heads/*:refs/remotes/origin/*"
+'
+
 test_done