git-rebase: print name of rev when using shorthand

The output from a successful invocation of the shorthand command
"git rebase -" is something like "Fast-forwarded HEAD to @{-1}",
which includes a relative reference to a revision. Other commands
that use the shorthand "-", such as "git checkout -", typically
display the symbolic name of the revision.

Change rebase to output the symbolic name of the revision when using
the shorthand. For the example above, the new output is
"Fast-forwarded HEAD to master", assuming "@{-1}" is a reference to
"master".

Requested-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Brian Gesiak <modocache@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>