diff.c: split emit_line() from the first char and the rest of the line

A new helper function emit_line_0() takes the first line of diff output
(typically "-", " ", or "+") separately from the remainder of the line.
No other functional changes.

This change will make it easier to reuse the logic when emitting the
rewrite diff, as we do not want to copy a line only to add "+"/"-"/" "
immediately before its first character when we produce rewrite diff
output.

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

diff.c

@@ -377,18 +377,31 @@ static void check_blank_at_eof(mmfile_t *mf1, mmfile_t *mf2,
 	ecbdata->blank_at_eof_in_postimage = (at - l2) + 1;
 }
 
-static void emit_line(FILE *file, const char *set, const char *reset, const char *line, int len)
+static void emit_line_0(FILE *file, const char *set, const char *reset,
+			int first, const char *line, int len)
 {
 	int has_trailing_newline, has_trailing_carriage_return;
+	int nofirst;
 
-	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--;
+	if (len == 0) {
+		has_trailing_newline = (first == '\n');
+		has_trailing_carriage_return = (!has_trailing_newline &&
+						(first == '\r'));
+		nofirst = has_trailing_newline || has_trailing_carriage_return;
+	} else {
+		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--;
+		nofirst = 0;
+	}
 
 	fputs(set, file);
+
+	if (!nofirst)
+		fputc(first, file);
 	fwrite(line, len, 1, file);
 	fputs(reset, file);
 	if (has_trailing_carriage_return)
@@ -397,6 +410,12 @@ static void emit_line(FILE *file, const char *set, const char *reset, const char
 		fputc('\n', file);
 }
 
+static void emit_line(FILE *file, const char *set, const char *reset,
+		      const char *line, int len)
+{
+	emit_line_0(file, set, reset, line[0], line+1, len-1);
+}
+
 static int new_blank_line_at_eof(struct emit_callback *ecbdata, const char *line, int len)
 {
 	if (!((ecbdata->ws_rule & WS_BLANK_AT_EOF) &&