diffcore-rename: fall back to -C when -C -C busts the rename limit

When there are too many paths in the project, the number of rename source
candidates "git diff -C -C" finds will exceed the rename detection limit,
and no inexact rename detection is performed.  We however could fall back
to "git diff -C" if the number of modified paths is sufficiently small.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/diff.c b/diff.c
index 9b3eb9938..13f322bc5 100644
--- a/diff.c
+++ b/diff.c
@@ -3956,6 +3956,28 @@ static int is_summary_empty(const struct diff_queue_struct *q)
 	return 1;
 }
 
+static const char rename_limit_warning[] =
+"inexact rename detection was skipped due to too many files.";
+
+static const char degrade_cc_to_c_warning[] =
+"only found copies from modified paths due to too many files.";
+
+static const char rename_limit_advice[] =
+"you may want to set your %s variable to at least "
+"%d and retry the command.";
+
+void diff_warn_rename_limit(const char *varname, int needed, int degraded_cc)
+{
+	if (degraded_cc)
+		warning(degrade_cc_to_c_warning);
+	else if (needed)
+		warning(rename_limit_warning);
+	else
+		return;
+	if (0 < needed && needed < 32767)
+		warning(rename_limit_advice, varname, needed);
+}
+
 void diff_flush(struct diff_options *options)
 {
 	struct diff_queue_struct *q = &diff_queued_diff;
@@ -4237,6 +4259,10 @@ void diffcore_std(struct diff_options *options)
 int diff_result_code(struct diff_options *opt, int status)
 {
 	int result = 0;
+
+	diff_warn_rename_limit("diff.renamelimit",
+			       opt->needed_rename_limit,
+			       opt->degraded_cc_to_c);
 	if (!DIFF_OPT_TST(opt, EXIT_WITH_STATUS) &&
 	    !(opt->output_format & DIFF_FORMAT_CHECKDIFF))
 		return status;