diff: Help "less" hide ^M from the output

When the tracked contents have CRLF line endings, colored diff output
shows "^M" at the end of output lines, which is distracting, even though
the pager we use by default ("less") knows to hide them.

The problem is that "less" hides a carriage-return only at the end of the
line, immediately before a line feed.  The colored diff output does not
take this into account, and emits four element sequence for each line:

   - force this color;
   - the line up to but not including the terminating line feed;
   - reset color
   - line feed.

By including the carriage return at the end of the line in the second
item, we are breaking the smart our pager has in order not to show "^M".
This can be fixed by changing the sequence to:

   - force this color;
   - the line up to but not including the terminating end-of-line;
   - reset color
   - end-of-line.

where end-of-line is either a single linefeed or a CRLF pair.  When the
output is not colored, "force this color" and "reset color" sequences are
both empty, so we won't have this problem with or without this patch.

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

1 files changed, 8 insertions, 1 deletions

diff --git a/diff.c b/diff.c
index 7b4300a74..6d56c6981 100644
--- a/diff.c
+++ b/diff.c
@@ -511,13 +511,20 @@ const char *diff_get_color(int diff_use_color, enum color_diff ix)
 
 static void emit_line(FILE *file, const char *set, const char *reset, const char *line, int len)
 {
-	int has_trailing_newline = (len > 0 && line[len-1] == '\n');
+	int has_trailing_newline, has_trailing_carriage_return;
+
+	has_trailing_newline = (len > 0 && line[len-1] == '\n');
 	if (has_trailing_newline)
 		len--;
+	has_trailing_carriage_return = (len > 0 && line[len-1] == '\r');
+	if (has_trailing_carriage_return)
+		len--;
 
 	fputs(set, file);
 	fwrite(line, len, 1, file);
 	fputs(reset, file);
+	if (has_trailing_carriage_return)
+		fputc('\r', file);
 	if (has_trailing_newline)
 		fputc('\n', file);
 }