rebase -i: write better reflog messages

Now that the "checkout" invoked internally from "rebase -i" knows to
honor GIT_REFLOG_ACTION, we can start to use it to write a better reflog
message when "rebase -i anotherbranch", "rebase -i --onto branch",
etc. internally checks out the new fork point.  We will write:

  rebase -i (start): checkout master

instead of the old

  rebase -i (start)

[jc: add rebase-reflog test]

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>