gitweb: Add author initials in 'blame' view, a la "git gui blame"

For example for "Junio C Hamano" initials would be "JH".  Of course
initials are added (below shortened SHA-1 of blamed commit) only if
group of lines that blame the same commit has 2 or more lines in it.

Initials are extracted using i18n /\b([[:upper:]])\B/g regexp.

Additionally initials help to distinguish boundary commits, as they
use bold weight font too (in addition to shortened SHA-1 of commit).

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>