git-gui (Windows): use git-bash.exe if it is available

git-gui (Windows): use git-bash.exe if it is available

Git for Windows 2.x ships with an executable that starts the Git Bash
with all the environment variables and what not properly set up. It is
also adjusted according to the Terminal emulator option chosen when
installing Git for Windows (while `bash.exe --login -i` would always
launch with Windows' default console).

So let's use that executable (usually C:\Program Files\Git\git-bash.exe)
instead of `bash.exe --login -i` if its presence was detected.

This fixes https://github.com/git-for-windows/git/issues/490

Signed-off-by: Thomas Kläger <thomas.klaeger@10a.ch>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>