Fix export of subdir of a branch.

Seems I forgot to fully test that feature when documenting it.

git rev-parse needs a colon after a branch to de-reference the tree
it points to, rather than the commit. But that had it adding an extra
colon when the user specified "branch:subdir". So, check if there is a
colon before adding one.

This commit was sponsored by Francois Marier on Patreon.

CHANGELOG

@@ -1,3 +1,9 @@
+git-annex (6.20171027) UNRELEASED; urgency=medium
+
+  * Fix export of subdir of a branch.
+
+ -- Joey Hess <id@joeyh.name>  Mon, 30 Oct 2017 12:01:45 -0400
+
 git-annex (6.20171026) unstable; urgency=medium
 
   * Windows: Fix reversion that caused the path used to link

Git/Ref.hs

@@ -136,8 +136,13 @@ delete oldvalue ref = run
 
 {- Gets the sha of the tree a ref uses. -}
 tree :: Ref -> Repo -> IO (Maybe Sha)
-tree ref = extractSha <$$> pipeReadStrict
-	[ Param "rev-parse", Param (fromRef ref ++ ":") ]
+tree (Ref ref) = extractSha <$$> pipeReadStrict
+	[ Param "rev-parse", Param ref' ]
+  where
+	ref' = if ':' `isInfixOf` ref
+		then ref
+		-- de-reference commit objects to the tree
+		else ref ++ ":"
 
 {- Checks if a String is a legal git ref name.
  -

doc/tips/publishing_your_files_to_the_public/comment_5_3216edeefa99f484194d6de061283170._comment

@@ -0,0 +1,8 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""Re: Problem with publishing subdir. Subdir does not exist, but it does."""
+ date="2017-10-30T15:53:33Z"
+ content="""
+I reproduced export of master:subdir not working. It's a bug in git-annex,
+now fixed!
+"""]]