tests: check availability of pyflakes by trying to import pyflakes module Since e397c6d74652, we use the pyflakes module instead of the pyflakes executable. As was pointed out during the review, the hghave check can be rewritten to try to import the pyflakes module instead of spawning a new subprocess.

nodemap: deal with the "debugupdatecache" case using a "fake" transaction We are going to need more and more methods of the transaction. So lets change approach. The `hg debugupdatecache` case do not need a transaction, because has the repositories locked, but is not adding any "store" data to it. Differential Revision: https://phab.mercurial-scm.org/D8186

changelog: change the implementation of `_divertopener` During a transaction, new data to the changelog index are "diverted". The previous implementat ways pretty basic, we need something better to be able to use the changelog vfs fully during transaction. (eg: accessing the vfs options). Differential Revision: https://phab.mercurial-scm.org/D8185

nodemap: test that an outdated nodemap can catch up If the persistent nodemap does not contains all revision that the index contains, this is should be detected and dealt with. We add a test for this case. Differential Revision: https://phab.mercurial-scm.org/D8183

nodemap: document the docket attributes More documentation rarely hurts. Differential Revision: https://phab.mercurial-scm.org/D8182

git: key off `git` in .hg/requires rather than separate file Differential Revision: https://phab.mercurial-scm.org/D8265

git: correctly handle p1() on dirstate when underlying git repo is empty This shows up in my next change, which ends up making an empty git repo and then running hg. Differential Revision: https://phab.mercurial-scm.org/D8271