added annex.commitmessage

Added annex.commitmessage config that can specify a commit message for the
git-annex branch instead of the usual "update".

This commit was supported by the NSF-funded DataLad project.

Command/Commit.hs

@@ -21,7 +21,7 @@ seek = withNothing start
 
 start :: CommandStart
 start = next $ next $ do
-	Annex.Branch.commit "update"
+	Annex.Branch.commit =<< Annex.Branch.commitMessage
 	_ <- runhook <=< inRepo $ Git.hookPath "annex-content"
 	return True
   where

Command/Merge.hs

@@ -27,7 +27,7 @@ mergeBranch = do
 	next $ do
 		Annex.Branch.update
 		-- commit explicitly, in case no remote branches were merged
-		Annex.Branch.commit "update"
+		Annex.Branch.commit =<< Annex.Branch.commitMessage
 		next $ return True
 
 mergeSynced :: CommandStart

Command/Sync.hs

@@ -301,7 +301,7 @@ commit :: SyncOptions -> CommandStart
 commit o = stopUnless shouldcommit $ next $ next $ do
 	commitmessage <- maybe commitMsg return (messageOption o)
 	showStart' "commit" Nothing
-	Annex.Branch.commit "update"
+	Annex.Branch.commit =<< Annex.Branch.commitMessage
 	ifM isDirect
 		( do
 			void stageDirect
@@ -544,7 +544,7 @@ pushBranch remote branch g = directpush `after` annexpush `after` syncpush
 
 commitAnnex :: CommandStart
 commitAnnex = do
-	Annex.Branch.commit "update"
+	Annex.Branch.commit =<< Annex.Branch.commitMessage
 	stop
 
 mergeAnnex :: CommandStart