import: add --bypass option
This feature is more a way to test patching without a working directory than
something people asked about. Adding a --rev option to specify the parent patch
revision would make it a little more useful.
What this change introduces is patch.repobackend class which let patches be
applied against repository revisions. The caller must supply a filestore object
to receive patched content, which can be turned into a memctx with
patch.makememctx() helper.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null