This menu option of Tools/Migrate has been living inside of git-gui
as a local hack to support some coworkers of mine. It has no value
to anyone outside of my day-job team and never really should have
been in a release version of git-gui. So I'm pulling it out, so
that nobody else has to deal with this garbage.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>