When annex.thin is set, allow hard links to be made between executable work tree files and annex objects.

This is safe, because while the annex object ends up executable,
there were already at least two other cases where it ended up executable:

1. git add an an executable file
2. chmod +x of a a non-executable worktree file that was hard linked to the
   annex object

After copy/hard link, it always fixes up the permissions to match the mode
of the worktree file, so when an executable annex object gets hard linked
to a non-executable worktree file, its execute bit gets removed.

Commit b7c8bf5274 already *said* it would do
this; I suspect the line of code I've removed was included in that commit
accidentially.

Also improves annex.thin documentation.

This commit was sponsored by Boyd Stephen Smith Jr. on Patreon.

Annex/Content/LowLevel.hs

@@ -49,7 +49,6 @@ linkOrCopy = linkOrCopy' (annexThin <$> Annex.getGitConfig)
 
 linkOrCopy' :: Annex Bool -> Key -> FilePath -> FilePath -> Maybe FileMode -> Annex (Maybe LinkedOrCopied)
 linkOrCopy' canhardlink key src dest destmode
-	| maybe False isExecutable destmode = copy =<< getstat
 	| otherwise = catchDefaultIO Nothing $
 		ifM canhardlink
 			( hardlink