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.
