gitweb: Mark boundary commits in 'blame' view

gitweb: Mark boundary commits in 'blame' view

Use "boundary" class to mark boundary commits, which currently results
in using bold weight font for SHA-1 of a commit (to be more exact for
all text in the first cell in row, that contains SHA-1 of a commit).

Detecting boundary commits is done by watching for "boundary" header
in "git blame -p" output.  Because this header doesn't carry
additional data the regular expression for blame header fields
had to be slightly adjusted.

With current gitweb API only root (parentless) commits can be boundary
commits.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>