annex.merge-annex-branches

Added annex.merge-annex-branches config setting which can be used to
disable automatic merge of git-annex branches.

I wonder if git-annex merge/sync/assistant should disable this
setting? Not sure yet, so have not done so. May be that users will not set
it in git config, but pass it via -c to commands that need it.

Checking the config setting adds a very small overhead, but it's
only checked once per command so should be insignificant.

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

Annex/Branch.hs

@@ -140,7 +140,13 @@ forceUpdate = updateTo =<< siblingBranches
  - Returns True if any refs were merged in, False otherwise.
  -}
 updateTo :: [(Git.Sha, Git.Branch)] -> Annex Bool
-updateTo pairs = do
+updateTo pairs = ifM (annexMergeAnnexBranches <$> Annex.getGitConfig)
+	( updateTo' pairs
+	, return False
+	)
+
+updateTo' :: [(Git.Sha, Git.Branch)] -> Annex Bool
+updateTo' pairs = do
 	-- ensure branch exists, and get its current ref
 	branchref <- getBranch
 	dirty <- journalDirty