support commit.gpgsign

Support users who have set commit.gpgsign, by disabling gpg signatures for
git-annex branch commits and commits made by the assistant.

The thinking here is that a user sets commit.gpgsign intending the commits
that they manually initiate to be gpg signed. But not commits made in the
background, whether by a deamon or implicitly to the git-annex branch.
gpg signing those would be at best a waste of CPU and at worst would fail,
or flood the user with gpg passphrase prompts, or put their signature on
changes they did not directly do.

See Debian bug #753720.

Also makes all commits done by git-annex go through a few central control
points, to make such changes easier in future.

Also disables commit.gpgsign in the test suite.

This commit was sponsored by Antoine Boegli.

Assistant/Threads/Committer.hs

@@ -221,7 +221,7 @@ commitStaged = do
 	case v of
 		Left _ -> return False
 		Right _ -> do
-			ok <- Command.Sync.commitStaged ""
+			ok <- Command.Sync.commitStaged Git.Branch.AutomaticCommit ""
 			when ok $
 				Command.Sync.updateSyncBranch =<< inRepo Git.Branch.current
 			return ok

Assistant/Threads/Merger.hs

@@ -83,7 +83,7 @@ onChange file
 				[ "merging", Git.fromRef changedbranch
 				, "into", Git.fromRef current
 				]
-			void $ liftAnnex  $ autoMergeFrom changedbranch (Just current)
+			void $ liftAnnex  $ autoMergeFrom changedbranch (Just current) Git.Branch.AutomaticCommit
 	mergecurrent _ = noop
 
 	handleDesynced = case fromTaggedBranch changedbranch of