git-gui: Use git web--browser for web browsing
This patch removes git-gui specific webbrowser guessing and instead
relies on git web--browser to do the right thing, removing unnecessary
code duplication. New function start_browser encapsulates the browser
execution, for usage from other parts of code. This will also make
git-gui show the documentation menu item even in cases it might not
be able to start up a browser, these cases should be however only
very rare.
Signed-off-by: Petr Baudis <petr.baudis@novartis.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>