blame: fix broken time_buf paddings in relative timestamp

Command `git blame --date relative` aligns the date field with a
fixed-width (defined by blame_date_width), and if time_str is shorter
than that, it adds spaces for padding.  But there are two bugs in the
following codes:

        time_len = strlen(time_str);
        ...
        memset(time_buf + time_len, ' ', blame_date_width - time_len);

 1. The type of blame_date_width is size_t, which is unsigned.  If
    time_len is greater than blame_date_width, the result of
    "blame_date_width - time_len" will never be a negative number, but a
    really big positive number, and will cause memory overwrite.

    This bug can be triggered if either l10n message for function
    show_date_relative() in date.c is longer than 30 characters, then
    `git blame --date relative` may exit abnormally.

 2. When show blame information with relative time, the UTF-8 characters
    in time_str will break the alignment of columns after the date field.
    This is because the time_buf padding with spaces should have a
    constant display width, not a fixed strlen size.  So we should call
    utf8_strwidth() instead of strlen() for width calibration.

Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jiangxin

builtin/blame.c

@@ -1556,22 +1556,29 @@ static void assign_blame(struct scoreboard *sb, int opt)
 static const char *format_time(unsigned long time, const char *tz_str,
 			       int show_raw_time)
 {
-	static char time_buf[128];
+	static struct strbuf time_buf = STRBUF_INIT;
 
+	strbuf_reset(&time_buf);
 	if (show_raw_time) {
-		snprintf(time_buf, sizeof(time_buf), "%lu %s", time, tz_str);
+		strbuf_addf(&time_buf, "%lu %s", time, tz_str);
 	}
 	else {
 		const char *time_str;
-		int time_len;
+		size_t time_width;
 		int tz;
 		tz = atoi(tz_str);
 		time_str = show_date(time, tz, blame_date_mode);
-		time_len = strlen(time_str);
-		memcpy(time_buf, time_str, time_len);
-		memset(time_buf + time_len, ' ', blame_date_width - time_len);
+		strbuf_addstr(&time_buf, time_str);
+		/*
+		 * Add space paddings to time_buf to display a fixed width
+		 * string, and use time_width for display width calibration.
+		 */
+		for (time_width = utf8_strwidth(time_str);
+		     time_width < blame_date_width;
+		     time_width++)
+			strbuf_addch(&time_buf, ' ');
 	}
-	return time_buf;
+	return time_buf.buf;
 }
 
 #define OUTPUT_ANNOTATE_COMPAT	001