New annex.supportunlocked config

Can beet to false to avoid some expensive things needed to support unlocked
files.

See my comment for why this only controls what init sets up, and not other
behavior.

I didn't bother with making the v5 upgrade code path look at this, though
it easily could, because the docs say to run git-annex init after setting
it to make it take effect.

Types/GitConfig.hs

@@ -128,6 +128,7 @@ data GitConfig = GitConfig
 	, annexCommitMode :: CommitMode
 	, annexSkipUnknown :: Bool
 	, annexAdjustedBranchRefresh :: Integer
+	, annexSupportUnlocked :: Bool
 	, coreSymlinks :: Bool
 	, coreSharedRepository :: SharedRepository
 	, receiveDenyCurrentBranch :: DenyCurrentBranch
@@ -229,6 +230,7 @@ extractGitConfig configsource r = GitConfig
 		-- parse as bool if it's not a number
 		(if getbool "adjustedbranchrefresh" False then 1 else 0)
 		(getmayberead (annexConfig "adjustedbranchrefresh"))
+	, annexSupportUnlocked = getbool (annexConfig "supportunlocked") True
 	, coreSymlinks = getbool "core.symlinks" True
 	, coreSharedRepository = getSharedRepository r
 	, receiveDenyCurrentBranch = getDenyCurrentBranch r