pass -c option on to all git commands run

The -c option now not only modifies the git configuration seen by
git-annex, but it is passed along to every git command git-annex runs.

This was easy to plumb through because gitCommandLine is already used to
construct every git command line, to add --git-dir and --work-tree

Git/Construct.hs

@@ -230,6 +230,7 @@ newFrom l = return Repo
 	, remotes = []
 	, remoteName = Nothing
 	, gitEnv = Nothing
+	, gitGlobalOpts = []
 	}