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>

combine-diff.c

@@ -500,6 +500,18 @@ static int hunk_comment_line(const char *bol)
 	return (isalpha(ch) || ch == '_' || ch == '$');
 }
 
+static void show_line_to_eol(const char *line, int len, const char *reset)
+{
+	int saw_cr_at_eol = 0;
+	if (len < 0)
+		len = strlen(line);
+	saw_cr_at_eol = (len && line[len-1] == '\r');
+
+	printf("%.*s%s%s\n", len - saw_cr_at_eol, line,
+	       reset,
+	       saw_cr_at_eol ? "\r" : "");
+}
+
 static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 		       int use_color)
 {
@@ -593,7 +605,7 @@ static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 					else
 						putchar(' ');
 				}
-				printf("%s%s\n", ll->line, c_reset);
+				show_line_to_eol(ll->line, -1, c_reset);
 				ll = ll->next;
 			}
 			if (cnt < lno)
@@ -617,7 +629,7 @@ static void dump_sline(struct sline *sline, unsigned long cnt, int num_parent,
 					putchar(' ');
 				p_mask <<= 1;
 			}
-			printf("%.*s%s\n", sl->len, sl->bol, c_reset);
+			show_line_to_eol(sl->bol, sl->len, c_reset);
 		}
 	}
 }

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);
 }

t/t4019-diff-wserror.sh

@@ -178,4 +178,16 @@ test_expect_success 'trailing empty lines (2)' '
 
 '
 
+test_expect_success 'do not color trailing cr in context' '
+	git config --unset core.whitespace
+	rm -f .gitattributes &&
+	echo AAAQ | tr Q "\015" >G &&
+	git add G &&
+	echo BBBQ | tr Q "\015" >>G
+	git diff --color G | tr "\015" Q >output &&
+	grep "BBB.*${blue_grep}Q" output &&
+	grep "AAA.*\[mQ" output
+
+'
+
 test_done