format-patch -n: make sorting easier by padding number

Now, when format-patch outputs more than 9 patches, the numbers
are padded accordingly. Example:

	[PATCH 009/167] The 9th patch of a series of 167

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

log-tree.c

@@ -102,6 +102,16 @@ static int append_signoff(char *buf, int buf_sz, int at, const char *signoff)
 	return at;
 }
 
+static unsigned int digits_in_number(unsigned int number)
+{
+	unsigned int i = 10, result = 1;
+	while (i <= number) {
+		i *= 10;
+		result++;
+	}
+	return result;
+}
+
 void show_log(struct rev_info *opt, const char *sep)
 {
 	static char this_header[16384];
@@ -155,7 +165,8 @@ void show_log(struct rev_info *opt, const char *sep)
 		if (opt->total > 0) {
 			static char buffer[64];
 			snprintf(buffer, sizeof(buffer),
-					"Subject: [PATCH %d/%d] ",
+					"Subject: [PATCH %0*d/%d] ",
+					digits_in_number(opt->total),
 					opt->nr, opt->total);
 			subject = buffer;
 		} else if (opt->total == 0)