improve smudge --clean behavior on outside work tree files

smudge: When passed a file located outside the working tree, eg by git
diff, avoid erroring out.

This commit was sponsored by Ewen McNeill on Patreon.

doc/bugs/v6_smudge_filter_breaks_git_diff_for_files_outside_the_repository.mdwn

@@ -22,3 +22,5 @@ $ git diff ../file1.txt ../file2.txt
 ### What version of git-annex are you using? On what operating system?
 
 6.20180913-1~bpo9+1 on Debian Stretch + backports.
+
+> [[fixed|done]] --[[Joey]]