web--browse: use custom commands defined at config time

web--browse: use custom commands defined at config time

Currently "git web--browse" is restricted to a set of commands defined
in the script. You can subvert the "browser.<tool>.path" to force "git
web--browse" to use a different command, but if you have a command
whose invocation syntax does not match one of the current tools then
you would have to write a wrapper script for it.

This patch adds a git config variable "browser.<tool>.cmd" which
allows a more flexible browser choice.

If you run "git web--browse" with -t/--tool, -b/--browser or the
"web.browser" config variable set to an unrecognized tool then "git
web--browse" will query the "browser.<tool>.cmd" config variable. If
this variable exists, then "git web--browse" will treat the specified
tool as a custom command and will use a shell eval to run the command
with the URLs added as extra parameters.

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