range-diff: left-pad patch numbers

As pointed out by Elijah Newren, tbdiff has this neat little alignment
trick where it outputs the commit pairs with patch numbers that are
padded to the maximal patch number's width:

	  1: cafedead =   1: acefade first patch
	[...]
	314: beefeada < 314: facecab up to PI!

Let's do the same in range-diff, too.

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

range-diff.c

@@ -259,6 +259,7 @@ static void get_correspondences(struct string_list *a, struct string_list *b,
 }
 
 static void output_pair_header(struct diff_options *diffopt,
+			       int patch_no_width,
 			       struct strbuf *buf,
 			       struct strbuf *dashes,
 			       struct patch_util *a_util,
@@ -295,9 +296,9 @@ static void output_pair_header(struct diff_options *diffopt,
 	strbuf_reset(buf);
 	strbuf_addstr(buf, status == '!' ? color_old : color);
 	if (!a_util)
-		strbuf_addf(buf, "-:  %s ", dashes->buf);
+		strbuf_addf(buf, "%*s:  %s ", patch_no_width, "-", dashes->buf);
 	else
-		strbuf_addf(buf, "%d:  %s ", a_util->i + 1,
+		strbuf_addf(buf, "%*d:  %s ", patch_no_width, a_util->i + 1,
 			    find_unique_abbrev(&a_util->oid, DEFAULT_ABBREV));
 
 	if (status == '!')
@@ -307,9 +308,9 @@ static void output_pair_header(struct diff_options *diffopt,
 		strbuf_addf(buf, "%s%s", color_reset, color_new);
 
 	if (!b_util)
-		strbuf_addf(buf, " -:  %s", dashes->buf);
+		strbuf_addf(buf, " %*s:  %s", patch_no_width, "-", dashes->buf);
 	else
-		strbuf_addf(buf, " %d:  %s", b_util->i + 1,
+		strbuf_addf(buf, " %*d:  %s", patch_no_width, b_util->i + 1,
 			    find_unique_abbrev(&b_util->oid, DEFAULT_ABBREV));
 
 	commit = lookup_commit_reference(the_repository, oid);
@@ -357,6 +358,7 @@ static void output(struct string_list *a, struct string_list *b,
 		   struct diff_options *diffopt)
 {
 	struct strbuf buf = STRBUF_INIT, dashes = STRBUF_INIT;
+	int patch_no_width = decimal_width(1 + (a->nr > b->nr ? a->nr : b->nr));
 	int i = 0, j = 0;
 
 	/*
@@ -378,23 +380,23 @@ 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(diffopt,
+			output_pair_header(diffopt, patch_no_width,
 					   &buf, &dashes, a_util, NULL);
 			i++;
 			continue;
 		}
 
 		/* Show unmatched RHS commits. */
 		while (j < b->nr && b_util->matching < 0) {
-			output_pair_header(diffopt,
+			output_pair_header(diffopt, patch_no_width,
 					   &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(diffopt,
+			output_pair_header(diffopt, patch_no_width,
 					   &buf, &dashes, a_util, b_util);
 			if (!(diffopt->output_format & DIFF_FORMAT_NO_OUTPUT))
 				patch_diff(a->items[b_util->matching].string,