On Windows, avoid git-gui to call Cygwin's nice utility

On Windows, avoid git-gui to call Cygwin's nice utility

It's a common case for Windows developers to have both Cygwin and msysGit
installed. Unfortunately, some scenarios also require to have Cygwin in PATH.
By default, Cygwin comes with nice.exe, while msysGit does not. Since git-gui
calls nice if it is in PATH, this results in Cygwin's nice.exe being called
from msysGit's git-gui. Mixing Cygwin and msysGit generally is not a good idea,
and in this particular case it causes differences not being correctly detected.
So we only call nice.exe on Windows if it is in the same directory as git.exe.
This way, this work-around does neither affect a pure Cygwin environment, or
the case when nice.exe will be shipped with msysGit at some point in time.

This fixes msysGit issue 394.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>