This makes possible to use unionrevlog class with subclasses of revlog that
override revlog's 'revision' and 'revdiff' methods. In particular this change
is necessary to implement manifest compression, as it allows extension to
replace manifest class and override 'revision' amd 'revdiff' methods there.
unionrepo is just like bundlerepo without bundles.
The implementation is very similar to bundlerepo, but I don't see any obvious
way to generalize it.
Some most obvious use cases for this would be log and diff across local repos,
as a kind of preview of pulls, for instance:
$ hg -R union:repo1+repo2 heads
$ hg -R union:repo1+repo2 log -r REPO1REV -r REPO2REV
$ hg -R union:repo1+repo2 log -r '::REPO1REV-::REPO2REV'
$ hg -R union:repo1+repo2 log -r 'ancestor(REPO1REV,REPO2REV)'
$ hg -R union:repo1+repo2 diff -r REPO1REV -r REPO2REV
This is going to be used in RhodeCode, and Bitbucket already uses something
similar. Having a core implementation would be beneficial.
