annex.autocommit can be configured via git-annex config

... to control the default behavior in all clones of a repository.

This includes a new Configurable data type, so the GitConfig type indicates
which values can be configured this way.

The implementation should be quite efficient; the config log is only read
once, and only when a Configurable value has not already been set by
git-config.

Indeed, it would be nice in the future to extend this, so that git-config
is itself only read on demand. Some commands may not need to look at the
git configuration at all.

This commit was sponsored by Trenton Cronholm on Patreon.

Config/GitConfig.hs

@@ -0,0 +1,36 @@
+{- git-annex configuration
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Config.GitConfig where
+
+import Annex.Common
+import qualified Annex
+import Types.GitConfig
+import Git.Types
+import Logs.Config
+
+{- Gets a specific setting from GitConfig. If necessary, loads the
+ - repository-global defaults when the GitConfig does not yet 
+ - have a value. -}
+getGitConfigVal :: (GitConfig -> Configurable a) -> Annex a
+getGitConfigVal f = do
+	v <- f <$> Annex.getGitConfig
+	case v of
+		HasConfig c -> return c
+		DefaultConfig _ -> do
+			r <- Annex.gitRepo
+			m <- loadGlobalConfig
+			let globalgc = extractGitConfig (r { config = m })
+			-- This merge of the repo-global config and the git
+			-- config makes all repository-global default
+			-- values populate the GitConfig with HasConfig
+			-- values, so it will only need to be done once.
+			Annex.changeGitConfig (\gc -> mergeGitConfig gc globalgc)
+			v' <- f <$> Annex.getGitConfig
+			case v' of
+				HasConfig c -> return c
+				DefaultConfig d -> return d