git-gui: Offer quick access to the HTML formatted documentation.

git-gui: Offer quick access to the HTML formatted documentation.

Users may want to be able to read Git documentation, even if they
are not command line users.  There are many important concepts and
terms covered within the standard Git documentation which would be
useful to even non command line using people.

We now try to offer an 'Online Documentation' menu option within the
Help menu.  First we try to guess to see what browser the user has
setup.  We default to instaweb.browser, if set, as this is probably
accurate for the user's configuration.  If not then we try to guess
based on the operating system and the available browsers for each.
We prefer documentation which is installed parallel to Git's own
executables, e.g. `git --exec-path`/../Documentation/index.html, as
that is how I typically install the HTML docs.  If those are not found
then we open the documentation published on kernel.org.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>