relax annex-tracking-branch to allow "/"

Allow setting remote.foo.annex-tracking-branch to a branch name that
contains "/", as long as it's not a remote tracking branch.

CHANGELOG

@@ -1,6 +1,8 @@
 git-annex (10.20250116) UNRELEASED; urgency=medium
 
   * Support help.autocorrect settings "prompt", "never", and "immediate".
+  * Allow setting remote.foo.annex-tracking-branch to a branch name
+    that contains "/", as long as it's not a remote tracking branch.
 
  -- Joey Hess <id@joeyh.name>  Mon, 20 Jan 2025 10:24:51 -0400
 

Command/Export.hs

@@ -120,15 +120,21 @@ seekExport r tree mtbcommitsha srcrs = do
 -- branch.
 getExportCommit :: Remote -> Git.Ref -> Annex (Maybe (RemoteTrackingBranch, Sha))
 getExportCommit r treeish
-	| '/' `notElem` fromRef baseref = do
+	| '/' `notElem` fromRef baseref = go
-		let tb = mkRemoteTrackingBranch r baseref
+	| otherwise = ifM isremoteref
-		commitsha <- inRepo $ Git.Ref.sha $ Git.Ref.underBase refsheads baseref
+		( return Nothing
-		return (fmap (tb, ) commitsha)
+		, go
-	| otherwise = return Nothing
+		)
   where
 	baseref = Ref $ S8.takeWhile (/= ':') $ fromRef' $ 
 		Git.Ref.removeBase refsheads treeish
 	refsheads = "refs/heads"
+	isremoteref = inRepo $ Git.Ref.exists $
+		Git.Ref.underBase "refs/remotes" baseref
+	go = do
+		let tb = mkRemoteTrackingBranch r baseref
+		commitsha <- inRepo $ Git.Ref.sha $ Git.Ref.underBase refsheads baseref
+		return (fmap (tb, ) commitsha)
 
 -- | Changes what's exported to the remote. Does not upload any new
 -- files, but does delete and rename files already exported to the remote.

doc/bugs/Can__39__t_export_synced__47___branches.mdwn

@@ -66,3 +66,5 @@ export myexport bla ok
 ### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
 
 git-annex is amazing, thank you! 🤩
+
+> [[fixed|done]] --[[Joey]]

doc/bugs/Can__39__t_export_synced__47___branches/comment_1_19ef18864eef6d08dae48df6ddb7152d._comment

@@ -0,0 +1,19 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2025-01-20T15:13:05Z"
+ content="""
+The difficulty here is that it needs to come up with the name of a
+corresponding remote tracking branch to update. It could use 
+"myexport/synced/main" for that. But, setting `annex-tracking-branch` to
+a remote ref like "origin/main" is not supported, and it would not be good
+if it allowed it and used a tracking branch with a name like
+"myexport/origin/main".
+
+And well, we know that "synced/main" is not a remote ref, but "$foo/main"
+generally may or may not be one.
+
+So, I think to support this, it would have only allow using "$foo/main"
+when the ref "refs/remotes/$foo/main" does not exist. I've implemented
+that.
+"""]]