git-rebase--merge: don't include absent parent as a base

git-rebase--merge: don't include absent parent as a base

Absent this fix, attempts to rebase an orphan branch using "rebase -m"
fails with:

    $ git rebase -m ORPHAN_TARGET_BASE
    First, rewinding head to replay your work on top of it...
    fatal: Could not parse object 'ORPHAN_ROOT_SHA^'
    Unknown exit code (128) from command: git-merge-recursive ORPHAN_ROOT_SHA^ -- HEAD ORPHAN_ROOT_SHA

To fix, this will only include the rebase root's parent as a base if it exists,
so that in cases of rebasing an orphan branch, it is a simple two-way merge.

Note the default rebase behavior does not fail:

    $ git rebase ORPHAN_TARGET_BASE
    First, rewinding head to replay your work on top of it...
    Applying: ORPHAN_ROOT_COMMIT_MSG
    Using index info to reconstruct a base tree...

A few tests were expecting the old behaviour to forbid rebasing such
a history with "rebase -m", which now need to expect them to succeed.

Signed-off-by: Ben Woosley <ben.woosley@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>