merge default into stable for 2.0 code freeze

rebase: simplify check for orphaned descendants

rebase: simplify set generation

rebase: add --rev option to rebase This option allow a strict set of revision to be specified instead of using -s or -b. Rebase will refuse start if striping rebased changeset will strip non rebased changeset. Rebase will refuse to work on set with multiple root.

rebase: allow revsets for source and base args

revset: fix %r handling in formatspec

rebase: use revset as soon as possible in internal logic The buildstate function now take a set of revs. Logic related to --source and --base option have been moved in the main rebase function. In the process this fixes a bug where the wrong source changeset might be pick. This explain the changes in hgext/rebase.py

revset: add %r for embedded revset support to formatspec This allows folding external revsets or lists of revsets into a revset expression. Revsets are pre-parsed for validity so that syntax errors don't escape.

revert: warn that subrepos cannot be reverted (tests added by mpm)

merge: expand environment variables and ~/ in tool.executable hgrc(5) already implies that this works, so we might as well support it. Another approach would be to implement this in util.findexe(): that would benefit other callers of findexe(), e.g. convert and anyone calling the user's editor. But findexe() is really implemented in both posix.py and windows.py, so this would make both of those modules depend on util.py: not good. So keep it narrow and only for merge tools.