work around lack of receive.denyCurrentBranch in direct mode

Now that direct mode sets core.bare=true, git's normal prohibition about
pushing into the currently checked out branch doesn't work.

A simple fix for this would be an update hook which blocks the pushes..
but git hooks must be executable, and git-annex needs to be usable on eg,
FAT, which lacks x bits.

Instead, enabling direct mode switches the branch (eg master) to a special
purpose branch (eg annex/direct/master). This branch is not pushed when
syncing; instead any changes that git annex sync commits get written to
master, and it's pushed (along with synced/master) to the remote.

Note that initialization has been changed to always call setDirect,
even if it's just setDirect False for indirect mode. This is needed because
if the user has just cloned a direct mode repo, that nothing has synced
with before, it may have no master branch, and only a annex/direct/master.
Resulting in that branch being checked out locally too. Calling setDirect False
for indirect mode moves back out of this branch, to a new master branch,
and ensures that a manual "git push" doesn't push changes directly to
the annex/direct/master of the remote. (It's possible that the user
makes a commit w/o using git-annex and pushes it, but nothing I can do
about that really.)

This commit was sponsored by Jonathan Harrington.

Git/Branch.hs

@@ -13,7 +13,7 @@ import Common
 import Git
 import Git.Sha
 import Git.Command
-import Git.Ref (headRef)
+import qualified Git.Ref
 
 {- The currently checked out branch.
  -
@@ -36,7 +36,7 @@ current r = do
 {- The current branch, which may not really exist yet. -}
 currentUnsafe :: Repo -> IO (Maybe Git.Ref)
 currentUnsafe r = parse . firstLine
-	<$> pipeReadStrict [Param "symbolic-ref", Param $ show headRef] r
+	<$> pipeReadStrict [Param "symbolic-ref", Param $ show Git.Ref.headRef] r
   where
 	parse l
 		| null l = Nothing
@@ -113,3 +113,21 @@ update branch sha = run
 	, Param $ show branch
 	, Param $ show sha
 	]
+
+{- Checks out a branch, creating it if necessary. -}
+checkout :: Branch -> Repo -> IO ()
+checkout branch = run
+	[ Param "checkout"
+	, Param "-q"
+	, Param "-B"
+	, Param $ show $ Git.Ref.base branch
+	]
+
+{- Removes a branch. -}
+delete :: Branch -> Repo -> IO ()
+delete branch = run
+	[ Param "branch"
+	, Param "-q"
+	, Param "-D"
+	, Param $ show $ Git.Ref.base branch
+	]

Git/Ref.hs

@@ -29,11 +29,17 @@ base = Ref . remove "refs/heads/" . remove "refs/remotes/" . show
 		| prefix `isPrefixOf` s = drop (length prefix) s
 		| otherwise = s
 
+{- Given a directory and any ref, takes the basename of the ref and puts
+ - it under the directory. -}
+under :: String -> Ref -> Ref
+under dir r = Ref $ dir ++ "/" ++
+	(reverse $ takeWhile (/= '/') $ reverse $ show r)
+
 {- Given a directory such as "refs/remotes/origin", and a ref such as
  - refs/heads/master, yields a version of that ref under the directory,
  - such as refs/remotes/origin/master. -}
-under :: String -> Ref -> Ref
-under dir r = Ref $ dir </> show (base r)
+underBase :: String -> Ref -> Ref
+underBase dir r = Ref $ dir ++ "/" ++ show (base r)
 
 {- Checks if a ref exists. -}
 exists :: Ref -> Repo -> IO Bool