range-diff: use color for the commit pairs

Arguably the most important part of `git range-diff`'s output is the
list of commits in the two branches, together with their relationships.

For that reason, tbdiff introduced color-coding that is pretty
intuitive, especially for unchanged patches (all dim yellow, like the
first line in `git show`'s output) vs modified patches (old commit is
red, new commit is green). Let's imitate that color scheme.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

range-diff.c

@@ -258,34 +258,53 @@ static void get_correspondences(struct string_list *a, struct string_list *b,
 	free(b2a);
 }
 
-static void output_pair_header(struct strbuf *buf,
+static void output_pair_header(struct diff_options *diffopt,
+			       struct strbuf *buf,
 			       struct strbuf *dashes,
 			       struct patch_util *a_util,
 			       struct patch_util *b_util)
 {
 	struct object_id *oid = a_util ? &a_util->oid : &b_util->oid;
 	struct commit *commit;
+	char status;
+	const char *color_reset = diff_get_color_opt(diffopt, DIFF_RESET);
+	const char *color_old = diff_get_color_opt(diffopt, DIFF_FILE_OLD);
+	const char *color_new = diff_get_color_opt(diffopt, DIFF_FILE_NEW);
+	const char *color_commit = diff_get_color_opt(diffopt, DIFF_COMMIT);
+	const char *color;
 
 	if (!dashes->len)
 		strbuf_addchars(dashes, '-',
 				strlen(find_unique_abbrev(oid,
 							  DEFAULT_ABBREV)));
 
+	if (!b_util) {
+		color = color_old;
+		status = '<';
+	} else if (!a_util) {
+		color = color_new;
+		status = '>';
+	} else if (strcmp(a_util->patch, b_util->patch)) {
+		color = color_commit;
+		status = '!';
+	} else {
+		color = color_commit;
+		status = '=';
+	}
+
 	strbuf_reset(buf);
+	strbuf_addstr(buf, status == '!' ? color_old : color);
 	if (!a_util)
 		strbuf_addf(buf, "-:  %s ", dashes->buf);
 	else
 		strbuf_addf(buf, "%d:  %s ", a_util->i + 1,
 			    find_unique_abbrev(&a_util->oid, DEFAULT_ABBREV));
 
-	if (!a_util)
+	if (status == '!')
-		strbuf_addch(buf, '>');
+		strbuf_addf(buf, "%s%s", color_reset, color);
-	else if (!b_util)
+	strbuf_addch(buf, status);
-		strbuf_addch(buf, '<');
+	if (status == '!')
-	else if (strcmp(a_util->patch, b_util->patch))
+		strbuf_addf(buf, "%s%s", color_reset, color_new);
-		strbuf_addch(buf, '!');
-	else
-		strbuf_addch(buf, '=');
 
 	if (!b_util)
 		strbuf_addf(buf, " -:  %s", dashes->buf);
@@ -295,10 +314,13 @@ static void output_pair_header(struct strbuf *buf,
 
 	commit = lookup_commit_reference(the_repository, oid);
 	if (commit) {
+		if (status == '!')
+			strbuf_addf(buf, "%s%s", color_reset, color);
+
 		strbuf_addch(buf, ' ');
 		pp_commit_easy(CMIT_FMT_ONELINE, commit, buf);
 	}
-	strbuf_addch(buf, '\n');
+	strbuf_addf(buf, "%s\n", color_reset);
 
 	fwrite(buf->buf, buf->len, 1, stdout);
 }
@@ -356,21 +378,24 @@ static void output(struct string_list *a, struct string_list *b,
 
 		/* Show unmatched LHS commit whose predecessors were shown. */
 		if (i < a->nr && a_util->matching < 0) {
-			output_pair_header(&buf, &dashes, a_util, NULL);
+			output_pair_header(diffopt,
+					   &buf, &dashes, a_util, NULL);
 			i++;
 			continue;
 		}
 
 		/* Show unmatched RHS commits. */
 		while (j < b->nr && b_util->matching < 0) {
-			output_pair_header(&buf, &dashes, NULL, b_util);
+			output_pair_header(diffopt,
+					   &buf, &dashes, NULL, b_util);
 			b_util = ++j < b->nr ? b->items[j].util : NULL;
 		}
 
 		/* Show matching LHS/RHS pair. */
 		if (j < b->nr) {
 			a_util = a->items[b_util->matching].util;
-			output_pair_header(&buf, &dashes, a_util, b_util);
+			output_pair_header(diffopt,
+					   &buf, &dashes, a_util, b_util);
 			if (!(diffopt->output_format & DIFF_FORMAT_NO_OUTPUT))
 				patch_diff(a->items[b_util->matching].string,
 					   b->items[j].string, diffopt);