git-gui: let nice work on Windows

git-gui: let nice work on Windows

git-gui runs blame and diff commands with nice by default. On Unix, nice
is accepted if found and it will run git. Commit ff9db6c79d ("On
Windows, avoid git-gui to call Cygwin's nice utility", 2010-10-05)
rejects nice if not collocated with git. In Git for Windows' (g4w) POSIX
path name space, nice and git are found in different directories:

	$ which git
	/mingw64/bin/git
	$ which nice
	/usr/bin/nice

Thus, git-gui will not use nice in the supported Windows configuration.
Commit ff9db6c79d justifies the collocation requirement as avoiding
problems in a mixed MSYS and Cygwin configuration: such configurations
are not supported by either project as they are known to cause many
problems.

So, let's revert ff9db6c79d and let git-gui work correctly in the
supported configuration.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>