Re: [PATCH v2 2/3] diff --submodule=diff: do not fail on ever-initialied deleted submodules

If you have ever initialized a submodule, open_submodule will open it.
If you then delete the submodule's worktree directory (but don't
remove it from .gitmodules), git diff --submodule=diff would error out
as it attempted to chdir into the now-deleted working tree directory.

Instead, we chdir into the submodule's git directory and run the diff
from there.

Signed-off-by: David Turner <redacted>
---
 submodule.c                                  |  10 ++
 t/t4060-diff-submodule-option-diff-format.sh | 157 +++++++++++++++++++++++++--
 2 files changed, 160 insertions(+), 7 deletions(-)

diff --git a/submodule.c b/submodule.c
index 0b1d9c1dde..d13d103975 100644
--- a/submodule.c
+++ b/submodule.c

@@ -710,6 +710,16 @@ void show_submodule_inline_diff(struct diff_options *o, const char *path,
 		strvec_push(&cp.args, oid_to_hex(new_oid));
 
 	prepare_submodule_repo_env(&cp.env_array);
+
+	if (!is_directory(path)) {
+		// fall back to absorbed git dir, if any

+		if (!sub)
+			goto done;
+		cp.dir = sub->gitdir;
+		strvec_push(&cp.env_array, GIT_DIR_ENVIRONMENT "=.");
+		strvec_push(&cp.env_array, GIT_WORK_TREE_ENVIRONMENT "=.");
+	}
+
 	if (start_command(&cp))
 		diff_emit_submodule_error(o, "(diff failed)\n");