blame: dynamic blame_date_width for different locales

When show date in relative date format for git-blame, the max display
width of datetime is set as the length of the string "Thu Oct 19
16:00:04 2006 -0700" (30 characters long).  But actually the max width
for C locale is only 22 (the length of string "x years, xx months ago").
And for other locale, it maybe smaller.  E.g. For Chinese locale, only
needs a half (16-character width).

Set blame_date_width as the display width of _("4 years, 11 months
ago"), so that translators can make the choice.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -2338,7 +2338,14 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 		blame_date_width = sizeof("2006-10-19");
 		break;
 	case DATE_RELATIVE:
-		/* "normal" is used as the fallback for "relative" */
+		/* TRANSLATORS: This string is used to tell us the maximum
+		   display width for a relative timestamp in "git blame"
+		   output.  For C locale, "4 years, 11 months ago", which
+		   takes 22 places, is the longest among various forms of
+		   relative timestamps, but your language may need more or
+		   fewer display columns. */
+		blame_date_width = utf8_strwidth(_("4 years, 11 months ago")) + 1; /* add the null */
+		break;
 	case DATE_LOCAL:
 	case DATE_NORMAL:
 		blame_date_width = sizeof("Thu Oct 19 16:00:04 2006 -0700");