submodule: unset core.worktree if no working tree is present

When a submodules work tree is removed, we should unset its core.worktree
setting as the worktree is no longer present. This is not just in line
with the conceptual view of submodules, but it fixes an inconvenience
for looking at submodules that are not checked out:

    git clone --recurse-submodules git://github.com/git/git && cd git &&
    git checkout --recurse-submodules v2.13.0
    git -C .git/modules/sha1collisiondetection log
    fatal: cannot chdir to '../../../sha1collisiondetection': \
        No such file or directory

With this patch applied, the final call to git log works instead of dying
in its setup, as the checkout will unset the core.worktree setting such
that following log will be run in a bare repository.

This patch covers all commands that are in the unpack machinery, i.e.
checkout, read-tree, reset. A follow up patch will address
"git submodule deinit", which will also make use of the new function
submodule_unset_core_worktree(), which is why we expose it in this patch.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -1532,6 +1532,18 @@ int bad_to_remove_submodule(const char *path, unsigned flags)
 	return ret;
 }
 
+void submodule_unset_core_worktree(const struct submodule *sub)
+{
+	char *config_path = xstrfmt("%s/modules/%s/config",
+				    get_git_common_dir(), sub->name);
+
+	if (git_config_set_in_file_gently(config_path, "core.worktree", NULL))
+		warning(_("Could not unset core.worktree setting in submodule '%s'"),
+			  sub->path);
+
+	free(config_path);
+}
+
 static const char *get_super_prefix_or_empty(void)
 {
 	const char *s = get_super_prefix();
@@ -1697,6 +1709,8 @@ int submodule_move_head(const char *path,
 
 			if (is_empty_dir(path))
 				rmdir_or_warn(path);
+
+			submodule_unset_core_worktree(sub);
 		}
 	}
 out:

submodule.h

@@ -121,6 +121,8 @@ extern int submodule_move_head(const char *path,
 			       const char *new_head,
 			       unsigned flags);
 
+void submodule_unset_core_worktree(const struct submodule *sub);
+
 /*
  * Prepare the "env_array" parameter of a "struct child_process" for executing
  * a submodule by clearing any repo-specific environment variables, but

t/lib-submodule-update.sh

@@ -709,7 +709,8 @@ test_submodule_recursing_with_args_common() {
 			git branch -t remove_sub1 origin/remove_sub1 &&
 			$command remove_sub1 &&
 			test_superproject_content origin/remove_sub1 &&
-			! test -e sub1
+			! test -e sub1 &&
+			test_must_fail git config -f .git/modules/sub1/config core.worktree
 		)
 	'
 	# ... absorbing a .git directory along the way.