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>