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