squash! git-gui: Use /git-bash.exe if available

Adjust commit message as follows:

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 #490

Signed-off-by: Thomas Kläger <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho