gitweb: Create Gitweb::Git module

gitweb: Create Gitweb::Git module

Create a Gitweb::Git module in  'gitweb/lib/Gitweb/Git.pm'
to deal with running git commands (and also processing output
of git commands with external programs) from gitweb.

This module is intended as standalone module, which does not require
(include) other gitweb' modules to avoid circular dependencies.  That
is why it includes $GIT variable, even though this variable is
configured during building gitweb.  On the other hand $GIT is more
about git configuration, than gitweb configuration.

Subroutines moved:
	evaluate_git_version
	git_cmd
	quote_command

Update gitweb/Makefile to install Gitweb::Git module alongside gitweb

Signed-off-by: Pavan Kumar Sunkara <pavan.sss1991@gmail.com>