repack -ad: prune the list of shallow commits

While it is true that we never add unreachable commits into pack files
intentionally (as `git repack`'s documentation states), we must not
forget that a `git fetch --prune` (or even a `git fetch` when a ref was
force-pushed in the meantime) can make a commit unreachable that was
reachable before.

Therefore it is not safe to assume that a `git repack -adlf` will keep
unreachable commits alone (under the assumption that they had not been
packed in the first place).

This is particularly important to keep in mind when looking at the
`.git/shallow` file: if any commits listed in that file become
unreachable, it is not a problem, but if they go missing, it *is* a
problem. One symptom of this problem is that a deepening fetch may now
fail with

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

To avoid this problem, let's prune the shallow list in `git repack` when
the `-d` option is passed, unless `-A` is passed, too (which would force
the now-unreachable objects to be turned into loose objects instead of
being deleted). Additionally, e also need to take `--keep-reachable` and
`--unpack-unreachable=<date>` into account.

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

Author: dscho

builtin/repack.c

@@ -444,6 +444,12 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
 		if (!quiet && isatty(2))
 			opts |= PRUNE_PACKED_VERBOSE;
 		prune_packed_objects(opts);
+
+		if (!keep_unreachable &&
+		    (!(pack_everything & LOOSEN_UNREACHABLE) ||
+		     unpack_unreachable) &&
+		    is_repository_shallow())
+			prune_shallow(0);
 	}
 
 	if (!no_update_server_info)

t/t5537-fetch-shallow.sh

@@ -186,7 +186,7 @@ EOF
 	test_cmp expect actual
 '
 
-test_expect_failure '.git/shallow is edited by repack' '
+test_expect_success '.git/shallow is edited by repack' '
 	git init shallow-server &&
 	test_commit -C shallow-server A &&
 	test_commit -C shallow-server B &&