rev-parse: avoid writing to const string for parent marks

The previous commit cleared up some const confusion in handling parent
marks in revision.c, but we have roughly the same code duplicated in
rev-parse. This one is much easier to fix, because the handling of the
shortened string is all done in one place, after detecting any marks
(but without shortening the string between marks).

  As a side note, I suspect this means that it behaves differently than
  the revision.c parser for weird stuff like "foo^!^@^-", but that is
  outside the scope of this patch.

While we are here, let's also rename the variable "dotdot", which is
totally misleading (and which we already fixed in revision.c long ago
via f632ded (handle_revision_arg: stop using "dotdot" as a generic
pointer, 2017-05-19)).

Doing that here makes the diff a little messier, but it also lets the
compiler help us make sure we did not miss any stray mentions of the
variable while we are changing its semantics.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

builtin/rev-parse.c

@@ -326,46 +326,47 @@ static int try_difference(const char *arg)
 
 static int try_parent_shorthands(const char *arg)
 {
-	char *dotdot;
+	const char *mark;
 	struct object_id oid;
 	struct commit *commit;
 	struct commit_list *parents;
 	int parent_number;
 	int include_rev = 0;
 	int include_parents = 0;
 	int exclude_parent = 0;
+	char *to_free;
 
-	if ((dotdot = strstr(arg, "^!"))) {
+	if ((mark = strstr(arg, "^!"))) {
 		include_rev = 1;
-		if (dotdot[2])
+		if (mark[2])
 			return 0;
-	} else if ((dotdot = strstr(arg, "^@"))) {
+	} else if ((mark = strstr(arg, "^@"))) {
 		include_parents = 1;
-		if (dotdot[2])
+		if (mark[2])
 			return 0;
-	} else if ((dotdot = strstr(arg, "^-"))) {
+	} else if ((mark = strstr(arg, "^-"))) {
 		include_rev = 1;
 		exclude_parent = 1;
 
-		if (dotdot[2]) {
+		if (mark[2]) {
 			char *end;
-			exclude_parent = strtoul(dotdot + 2, &end, 10);
+			exclude_parent = strtoul(mark + 2, &end, 10);
 			if (*end != '\0' || !exclude_parent)
 				return 0;
 		}
 	} else
 		return 0;
 
-	*dotdot = 0;
+	arg = to_free = xmemdupz(arg, mark - arg);
 	if (repo_get_oid_committish(the_repository, arg, &oid) ||
 	    !(commit = lookup_commit_reference(the_repository, &oid))) {
-		*dotdot = '^';
+		free(to_free);
 		return 0;
 	}
 
 	if (exclude_parent &&
 	    exclude_parent > commit_list_count(commit->parents)) {
-		*dotdot = '^';
+		free(to_free);
 		return 0;
 	}
 
@@ -386,7 +387,7 @@ static int try_parent_shorthands(const char *arg)
 		free(name);
 	}
 
-	*dotdot = '^';
+	free(to_free);
 	return 1;
 }