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>