bookmarks: introduce a repo._bookmarks.changectx(mark) method and use it
Many places were doing repo[mark], which usually works, but it's
slightly incorrect: if the bookmark has a name that matches a full hex
nodeid of another node, then the context for the other node will be
returned instead. Also, I'm about to remove support for
repo[<namespace thing>] :)
Differential Revision: https://phab.mercurial-scm.org/D3162
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.