parse_pathspec: support prefixing original patterns

This makes 'original' suitable for passing to an external command
because all pathspec magic is left in place, provided that the
external command understands pathspec. The prefixing is needed because
we usually launch a subcommand at worktree's top directory and the
subcommand can no longer calculate the prefix itself.

This slightly affects the original purpose of 'original'
(i.e. reporting). We should report without prefixing. So only turn
this flag on when you know you are about to pass the result straight
away to an external command.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pathspec.c

@@ -203,7 +203,17 @@ static unsigned prefix_pathspec(struct pathspec_item *item,
 	else
 		match = prefix_path(prefix, prefixlen, copyfrom);
 	*raw = item->match = match;
-	item->original = elt;
+	/*
+	 * Prefix the pathspec (keep all magic) and assign to
+	 * original. Useful for passing to another command.
+	 */
+	if (flags & PATHSPEC_PREFIX_ORIGIN) {
+		struct strbuf sb = STRBUF_INIT;
+		strbuf_add(&sb, elt, copyfrom - elt);
+		strbuf_addstr(&sb, match);
+		item->original = strbuf_detach(&sb, NULL);
+	} else
+		item->original = elt;
 	item->len = strlen(item->match);
 
 	if ((flags & PATHSPEC_STRIP_SUBMODULE_SLASH_CHEAP) &&

pathspec.h

@@ -43,6 +43,7 @@ struct pathspec {
  * safer than _SLASH_CHEAP and also more expensive.
  */
 #define PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE (1<<5)
+#define PATHSPEC_PREFIX_ORIGIN (1<<6)
 
 extern int init_pathspec(struct pathspec *, const char **);
 extern void parse_pathspec(struct pathspec *pathspec,