tests: fix breakage caused by Mercurial changes

Mercurial commit e4f451c8c05c added a new message to `hg resolve` with no more
unresolved files.
readme: drop Mercurial and Dulwich compatibilty information

No one's ever going to bother keeping this in sync.
testedwith: update list of versions hg-git is tested with

This should be kept in sync with the Makefile, but hasn't been.
tests: test with @ instead of tip
tests: assert compatibility with Mercurial 3.0
Fixed small typo