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>

git-gui.sh

@@ -638,8 +638,6 @@ proc _lappend_nice {cmd_var} {
 		set _nice [_which nice]
 		if {[catch {safe_exec [list $_nice git version]}]} {
 			set _nice {}
-		} elseif {[is_Windows] && [file dirname $_nice] ne [file dirname $::_git]} {
-			set _nice {}
 		}
 	}
 	if {$_nice ne {}} {