Add git-instaweb, instantly browse the working repo with gitweb

Add git-instaweb, instantly browse the working repo with gitweb

I got tired of having to configure gitweb for every repository
I work on.  I sometimes prefer gitweb to standard GUIs like gitk
or gitview; so this lets me automatically configure gitweb to
browse my working repository and also opens my browser to it.

Updates from the original patch:

Added Apache/mod_perl2 compatibility if Dennis Stosberg's gitweb
has been applied, too: <20060621130708.Gcbc6e5c@leonov.stosberg.net>

General cleanups in shell code usage.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>