WIP: git add -p: allow stashing hunks

In the default `git add -p` mode, users can selectively stash hunks.

In common work scenarios, I often encountered situations where I would
love to stash (or simply discard) hunks instead.

This idea has been brought up on the Git mailing list before, and it is
an idea worth putting effort behind.

This patch is utterly incomplete!

In particular, the reassemble-and-apply phase needs to be accompanied by
a preceding step that would reassemble the "to-stash" hunks and stash
them.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

add-patch.c

@@ -55,7 +55,8 @@ static struct patch_mode patch_mode_add = {
 			"ones\n"
 		   "a - stage this hunk and all later hunks in the file\n"
 		   "d - do not stage this hunk or any of the later hunks in "
-			"the file\n")
+			"the file\n"
+		   "t - stash this hunk instead of staging it\n")
 };
 
 static struct patch_mode patch_mode_stash = {
@@ -1215,7 +1216,7 @@ static int run_apply_check(struct add_p_state *s,
 			    "apply", "--check", NULL);
 	strvec_pushv(&cp.args, s->mode->apply_check_args);
 	if (pipe_command(&cp, s->buf.buf, s->buf.len, NULL, 0, NULL, 0))
-		return error(_("'git apply --cached' failed"));
+		return error(_("'git apply --check' failed"));
 
 	return 0;
 }
@@ -1697,6 +1698,8 @@ static int patch_update_file(struct add_p_state *s,
 		} else if (ch == 'p') {
 			rendered_hunk_index = -1;
 			use_pager = (s->answer.buf[0] == 'P') ? 1 : 0;
+		} else if (s->answer.buf[0] == 't') {
+			hunk->use = STASH_HUNK;
 		} else if (s->answer.buf[0] == '?') {
 			const char *p = _(help_patch_remainder), *eol = p;