refactor getCurrentBranch

Both Command.Sync and Annex.Ingest had their own versions of this.

The one in Annex.Ingest used Git.Branch.currentUnsafe, but does not seem
to need it. That is only checking to see if it's in an adjusted unlocked
branch, and when in an adjusted branch, the branch does in fact exist,
so the added check that Git.Branch.current does is fine.

This commit was sponsored by Denis Dzyubenko on Patreon.

Assistant/Threads/Exporter.hs

@@ -12,6 +12,7 @@ import Assistant.Commits
 import Assistant.Pushes
 import Assistant.DaemonStatus
 import Annex.Concurrent
+import Annex.CurrentBranch
 import Utility.ThreadScheduler
 import qualified Annex
 import qualified Remote
@@ -64,7 +65,7 @@ exportToRemotes rs = do
 			Annex.changeState $ \st -> st { Annex.errcounter = 0 }
 			start <- liftIO getCurrentTime
 			void $ Command.Sync.seekExportContent rs
-				=<< join Command.Sync.getCurrBranch
+				=<< getCurrentBranch
 			-- Look at command error counter to see if the export
 			-- didn't work.
 			failed <- (> 0) <$> Annex.getState Annex.errcounter