gitk: Fix enabling/disabling of menu items on Mac OS X

gitk: Fix enabling/disabling of menu items on Mac OS X

It seems that under Mac OS X, the menus get some extra entries (or
possibly fewer entries), leading to references to entries by an
absolute number being off.  This leads to an error when invoking
gitk --all under Mac OS X, because the "Edit view" and "Delete view"
entries aren't were gitk expects them, and so enabling them gives an
error.

This changes the code so it refers to menu entries by their content,
which should solve the problem.

Signed-off-by: Paul Mackerras <paulus@samba.org>