gitk: limit PATH search to bare executable names

The path search overrides used by gitk on Windows are applied to any executable whose name is not 'absolute', meaning that [exec foo/bar ...] will search each element of $PATH to find one with subdirectory foo containing bar. But, per POSIX, and Tcl implementation on all platforms, foo/bar is taken as $(pwd)/foo/bar, and is not searched on $PATH. Fix this descrepency using the same approach applied to git-gui in commit 3f71c97e. The key is that the executable name must have no path component, indicated by [file split $exename] having array length 1. Signed-off-by: Mark Levedahl <mlevedahl@gmail.com> Signed-off-by: Johannes Sixt <j6t@kdbg.org>