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
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3