gitk: Fix display of diff lines beginning with --- or +++

Lines in a diff beginning with --- or +++ were not being displayed
at all.  Thanks to Robert Fitzsimons for pointing out the obvious
fix, that lines beginning with --- or +++ are only to be suppressed
in the diff header.  I also took the opportunity to replace a regexp
call with a couple of string compare calls, which should be faster.

Signed-off-by: Paul Mackerras <paulus@samba.org>