git: ensure all dirstate state values are bytes

I'm not sure how this particular git status occurs, but after the fallout of
issue 6510 and getting into the issue 6511 state where `git status` shows the
files as modified in both the "to be committed" and "not staged" lists,
`hg diff` was crashing in `workingctx.__contains__()`.

Differential Revision: https://phab.mercurial-scm.org/D10532

hgext/git/dirstate.py

@@ -63,7 +63,7 b' if pygit2:'
         pygit2.GIT_STATUS_WT_RENAMED: b'a',
         pygit2.GIT_STATUS_WT_TYPECHANGE: b'n',
         pygit2.GIT_STATUS_WT_UNREADABLE: b'?',
-        pygit2.GIT_STATUS_INDEX_MODIFIED | pygit2.GIT_STATUS_WT_MODIFIED: 'm',
+        pygit2.GIT_STATUS_INDEX_MODIFIED | pygit2.GIT_STATUS_WT_MODIFIED: b'm',
     }