diff: factor out --follow pathspec check

In --follow mode, we require exactly one pathspec. We check this
condition in two places:

  - in diff_setup_done(), we complain if --follow is used with an
    inapropriate pathspec

  - in git-log's revision "tweak" function, we enable log.follow only if
    the pathspec allows it

The duplication isn't a big deal right now, since the logic is so
simple. But in preparation for it becoming more complex, let's pull it
into a shared function.

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

1 files changed, 12 insertions, 2 deletions

diff --git a/diff.c b/diff.c
index 469e18aed2..dc9f21a9dc 100644
--- a/diff.c
+++ b/diff.c
@@ -4721,6 +4721,16 @@ unsigned diff_filter_bit(char status)
 	return filter_bit[(int) status];
 }
 
+int diff_check_follow_pathspec(struct pathspec *ps, int die_on_error)
+{
+	if (ps->nr != 1) {
+		if (die_on_error)
+			die(_("--follow requires exactly one pathspec"));
+		return 0;
+	}
+	return 1;
+}
+
 void diff_setup_done(struct diff_options *options)
 {
 	unsigned check_mask = DIFF_FORMAT_NAME |
@@ -4828,8 +4838,8 @@ void diff_setup_done(struct diff_options *options)
 
 	options->diff_path_counter = 0;
 
-	if (options->flags.follow_renames && options->pathspec.nr != 1)
-		die(_("--follow requires exactly one pathspec"));
+	if (options->flags.follow_renames)
+		diff_check_follow_pathspec(&options->pathspec, 1);
 
 	if (!options->use_color || external_diff())
 		options->color_moved = 0;