git-gui: use `git --html-path` to get the location of installed HTML docs

git-gui: use `git --html-path` to get the location of installed HTML docs

Previously a hardcoded path $GIT_EXEC_PATH/../Documentation/ was used to
search for the documentation, when the user has asked for it via menu
"Help -> Online Documentation".
This didn't work for the default directory structure.

To find the path reliably, use the new git command line option, which
returns the correct path.

If the output of `git --html-path` is empty because git is not found or
the option is not yet supported in the installed git, the documentation
from kernel.org is launched. There is no additional guessing of the
right location of the installed docs.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>