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>