Rename ".dotest/" to ".git/rebase" and ".dotest-merge" to "rebase-merge"

Rename ".dotest/" to ".git/rebase" and ".dotest-merge" to "rebase-merge"

Since the files generated and used during a rebase are never to be
tracked, they should live in $GIT_DIR.  While at it, avoid the rather
meaningless term "dotest" to "rebase", and unhide ".dotest-merge".

This was wished for on the mailing list, but so far unimplemented.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>