dirstate: make map implementation overridable
Other implementations of dirstate will want to replace the class used for the
dirstate map. Allow this to happen by making the class an attribute of the
dirstate.
Differential Revision: https://phab.mercurial-scm.org/D1347
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..