gitweb: configurable home link string

gitweb: configurable home link string

I've always found difficult to figure out git URL for clone from
gitweb URL because git:// and http:// are different on many site
including kernel.org.

I've found this enhancement at http://dev.laptop.org/git when I was on
git channel, and thought that it'd be nice if all public gitweb site
show it's git URL on its page.

This patch allow us to change the home link string.  The current
default is "projects" as we all see on gitweb now.

ie. kernel.org might set this variable to "git://git.kernel.org/pub/scm/"

Signed-off-by: Yasushi SHOJI <redacted>
Signed-off-by: Junio C Hamano <redacted>