partially complete gcrypt remote (local send done; rest not)

This is a git-remote-gcrypt encrypted special remote. Only sending files
in to the remote works, and only for local repositories.

Most of the work so far has involved making initremote work. A particular
problem is that remote setup in this case needs to generate its own uuid,
derivied from the gcrypt-id. That required some larger changes in the code
to support.

For ssh remotes, this will probably just reuse Remote.Rsync's code, so
should be easy enough. And for downloading from a web remote, I will need
to factor out the part of Remote.Git that does that.

One particular thing that will need work is supporting hot-swapping a local
gcrypt remote. I think it needs to store the gcrypt-id in the git config of the
local remote, so that it can check it every time, and compare with the
cached annex-uuid for the remote. If there is a mismatch, it can change
both the cached annex-uuid and the gcrypt-id. That should work, and I laid
some groundwork for it by already reading the remote's config when it's
local. (Also needed for other reasons.)

This commit was sponsored by Daniel Callahan.

Remote/Helper/Git.hs

@@ -0,0 +1,30 @@
+{- Utilities for git remotes.
+ -
+ - Copyright 2011-2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Remote.Helper.Git where
+
+import Common.Annex
+import qualified Git
+
+repoCheap :: Git.Repo -> Bool
+repoCheap = not . Git.repoIsUrl
+
+localpathCalc :: Git.Repo -> Maybe FilePath
+localpathCalc r = if globallyAvailableCalc r
+	then Nothing
+	else Just $ Git.repoPath r
+
+globallyAvailableCalc :: Git.Repo -> Bool
+globallyAvailableCalc r = not $
+	Git.repoIsLocal r || Git.repoIsLocalUnknown r
+
+{- Avoids performing an action on a local repository that's not usable.
+ - Does not check that the repository is still available on disk. -}
+guardUsable :: Git.Repo -> a -> Annex a -> Annex a
+guardUsable r onerr a
+	| Git.repoIsLocalUnknown r = return onerr
+	| otherwise = a