Support .git/rebase-merge or .git/rebase-apply

The `$_rebase_base` magic variable determines the base commit of a rebase by
reading the metadata in the `.git` directory.

Rebase metadata is stored in different sub-directories at different times.
This commit adds support for `.git/rebase-merge` in addition to the existing
`.git/rebase-apply` support.

See: http://stackoverflow.com/questions/3921409