legacy stash -p: respect the add.interactive.usebuiltin setting

As `git add` traditionally did not expose the `--patch=<mode>` modes via
command-line options, the scripted version of `git stash` had to call
`git add--interactive` directly.

But this prevents the built-in `add -p` from kicking in, as
`add--interactive` is the scripted version (which does not have a
"fall-back" to the built-in version).

So let's introduce support for internal switch for `git add` that the
scripted `git stash` can use to call the appropriate backend (scripted
or built-in, depending on `add.interactive.useBuiltin`).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/add.c

@@ -29,6 +29,7 @@ static const char * const builtin_add_usage[] = {
 static int patch_interactive, add_interactive, edit_interactive;
 static int take_worktree_changes;
 static int add_renormalize;
+static int legacy_stash_p; /* support for the scripted `git stash` */
 
 struct update_callback_data {
 	int flags;
@@ -335,6 +336,8 @@ static struct option builtin_add_options[] = {
 		   N_("override the executable bit of the listed files")),
 	OPT_HIDDEN_BOOL(0, "warn-embedded-repo", &warn_on_embedded_repo,
 			N_("warn when adding an embedded repository")),
+	OPT_HIDDEN_BOOL(0, "legacy-stash-p", &legacy_stash_p,
+			N_("backend for `git stash -p`")),
 	OPT_END(),
 };
 
@@ -431,6 +434,17 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 		add_interactive = 1;
 	if (add_interactive)
 		exit(interactive_add(argc - 1, argv + 1, prefix, patch_interactive));
+	if (legacy_stash_p) {
+		struct pathspec pathspec;
+
+		parse_pathspec(&pathspec, 0,
+			PATHSPEC_PREFER_FULL |
+			PATHSPEC_SYMLINK_LEADING_PATH |
+			PATHSPEC_PREFIX_ORIGIN,
+			prefix, argv);
+
+		return run_add_interactive(NULL, "--patch=stash", &pathspec);
+	}
 
 	if (edit_interactive)
 		return(edit_patch(argc, argv, prefix));

git-legacy-stash.sh

@@ -206,7 +206,7 @@ create_stash () {
 
 		# find out what the user wants
 		GIT_INDEX_FILE="$TMP-index" \
-			git add--interactive --patch=stash -- "$@" &&
+			git add --legacy-stash-p -- "$@" &&
 
 		# state of the working tree
 		w_tree=$(GIT_INDEX_FILE="$TMP-index" git write-tree) ||