sync hack to make updateInstead work on eg FAT

sync: When syncing with a local repository located on a crippled
filesystem, run the post-receive hook there, since it wouldn't get run
otherwise. This makes pushing to repos on FAT-formatted removable drives
update them when receive.denyCurrentBranch=updateInstead.

Made Remote.Git export onLocal, which was cleaned up to not have so many
caveats about its use.

This commit was sponsored by Jeff Goeke-Smith on Patreon.

Annex/UpdateInstead.hs

@@ -0,0 +1,27 @@
+{- git-annex UpdateIntead emulation
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Annex.UpdateInstead where
+
+import qualified Annex
+import Annex.Common
+import Config
+import Annex.Version
+import Annex.AdjustedBranch
+import Git.Branch
+import Git.ConfigTypes
+
+{- receive.denyCurrentBranch=updateInstead does not work in direct mode
+ - repositories or when an adjusted branch is checked out, so must be
+ - emulated. -}
+needUpdateInsteadEmulation :: Annex Bool
+needUpdateInsteadEmulation = updateinsteadset <&&> (isDirect <||> isadjusted)
+  where
+	updateinsteadset = (== UpdateInstead) . receiveDenyCurrentBranch
+		<$> Annex.getGitConfig
+	isadjusted = versionSupportsUnlockedPointers
+		<&&> (maybe False (isJust . getAdjustment) <$> inRepo Git.Branch.current)