util: avoid using hashlib on Python < 2.5 (
issue2278)
The following patch allows the use of python2.4 with a standalone
hashlib rather than assuming that python2.5 is in use when hashlib is
imported successfully.
revset: predicate to avoid lookup errors
A query like
head() and (descendants("bad") and not descendants("fix"))
(testing if repo heads are affected by a bug) will abort with a
RepoLookupError if either badrev or fixrev aren't found inside
the repository, which is not very informative.
The new predicate returns an empty set for lookup errors, so
head() and (descendants(present("bad")) and not descendants(present("fix")))
will behave as wanted even if those revisions are not found.