rebase: reinstate old-style rev spec support for the source and base (issue3181)
As of b12362ab13e7 (first released as part of Mercurial 2.0), the rebase command
accepted ONLY revsets for the source and base arguments and no longer accepted
old-style revision specifications. As a result, some revision names were no
longer recognised, e.g.
hg rebase --base br-anch
abort: unknown revision 'br'!
These arguments are now interpreted first as old-style revision specifications,
then as revsets when no matching revision is found. This restores backwards
compatibility with releases prior to 2.0.