windows: enforce upper case drive letter for getcwd in mercurial too
This is affecting code that checks if a pull/push destination is the same as a
configured one. For example the one creating divergent bookmark.
Doing this fixes will help fixing test-bookmarks.t and test-bookflow.t on
Windows. However, we also need to fix abspath invocation.
Differential Revision: https://phab.mercurial-scm.org/D11058