gitk: Wait for the window to become visible after creating it

gitk: Wait for the window to become visible after creating it

When the git log process returned an error immediately, we were
sometimes getting no main window and no error window displayed,
with the gitk process just hanging waiting for something.  It appears
that the tkwait in show_error, which waits for the error window to
be destroyed, wasn't sufficient to allow the main window or the error
window to be mapped.

This adds a wait in the main startup code after the main window
has been created to wait until it is visible.  This seems to fix the
problem.

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