Added remote.<name>.annex-push and remote.<name>.annex-pull

The former can be useful to make remotes that don't get fully synced with
local changes, which comes up in a lot of situations.

The latter was mostly added for symmetry, but could be useful (though less
likely to be).

Implementing `remote.<name>.annex-pull` was a bit tricky, as there's no one
place where git-annex pulls/fetches from remotes. I audited all
instances of "fetch" and "pull". A few cases were left not checking this
config:

* Git.Repair can try to pull missing refs from a remote, and if the local
  repo is corrupted, that seems a reasonable thing to do even though
  the config would normally prevent it.
* Assistant.WebApp.Gpg and Remote.Gcrypt and Remote.Git do fetches
  as part of the setup process of a remote. The config would probably not
  be set then, and having the setup fail seems worse than honoring it if it
  is already set.

I have not prevented all the code that does a "merge" from merging branches
from remotes with remote.<name>.annex-pull=false. That could perhaps
be done, but it would need a way to map from branch name to remote name,
and the way refspecs work makes that hard to get really correct. So if the
user fetches manually, the git-annex branch will get merged, for example.
Anther way of looking at/justifying this is that the setting is called
"annex-pull", not "annex-merge".

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

Command/Sync.hs

@@ -360,7 +360,7 @@ updateBranch syncbranch updateto g =
 		] g
 
 pullRemote :: SyncOptions -> [Git.Merge.MergeConfig] -> Remote -> CurrBranch -> CommandStart
-pullRemote o mergeconfig remote branch = stopUnless (pure $ pullOption o) $ do
+pullRemote o mergeconfig remote branch = stopUnless (pure $ pullOption o && wantpull) $ do
 	showStart "pull" (Remote.name remote)
 	next $ do
 		showOutput
@@ -370,6 +370,7 @@ pullRemote o mergeconfig remote branch = stopUnless (pure $ pullOption o) $ do
 	fetch = inRepoWithSshOptionsTo (Remote.repo remote) (Remote.gitconfig remote) $
 		Git.Command.runBool
 			[Param "fetch", Param $ Remote.name remote]
+	wantpull = remoteAnnexPull (Remote.gitconfig remote)
 
 {- The remote probably has both a master and a synced/master branch.
  - Which to merge from? Well, the master has whatever latest changes
@@ -400,7 +401,7 @@ pushRemote o remote (Just branch, _) = stopUnless (pure (pushOption o) <&&> need
 	showStart "push" (Remote.name remote)
 	next $ next $ do
 		showOutput
-		ok <- inRepoWithSshOptionsTo (Remote.repo remote) (Remote.gitconfig remote) $
+		ok <- inRepoWithSshOptionsTo (Remote.repo remote) gc $
 			pushBranch remote branch
 		if ok
 			then postpushupdate
@@ -410,7 +411,8 @@ pushRemote o remote (Just branch, _) = stopUnless (pure (pushOption o) <&&> need
 				return ok
   where
 	needpush
-		| remoteAnnexReadOnly (Remote.gitconfig remote) = return False
+		| remoteAnnexReadOnly gc = return False
+		| not (remoteAnnexPush gc) = return False
 		| otherwise = anyM (newer remote) [syncBranch branch, Annex.Branch.name]
 	-- Do updateInstead emulation for remotes on eg removable drives
 	-- formatted FAT, where the post-update hook won't run.
@@ -426,6 +428,7 @@ pushRemote o remote (Just branch, _) = stopUnless (pure (pushOption o) <&&> need
 					, return True
 					)
 		| otherwise = return True
+	gc = Remote.gitconfig remote
 
 {- Pushes a regular branch like master to a remote. Also pushes the git-annex
  - branch.