git-help: add -w|--web option to display html man page in a browser.

git-help: add -w|--web option to display html man page in a browser.

Now when using "git help -w cmd", we will try to show the HTML man
page "git-cmd.html" in your prefered web browser.

To do that "help.c" code will call a new shell script
"git-browse-help".

This currently works only if the HTML versions of the man page
have been installed in $(htmldir) (typically "/usr/share/doc/git-doc"),
so new target to do that is added to "Documentation/Makefile".

The browser to use can be configured using the "web.browser"
config variable.

We try to open a new tab in an existing web browser, if possible.

The code in "git-browse-help" is heavily stolen from "git-mergetool"
by Theodore Y. Ts'o. Thanks.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>