Merge branch 'master' of ssh://git-annex.branchable.com

doc/bugs/v6_smudge_filter_breaks_git_diff_for_files_outside_the_repository.mdwn

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+### Please describe the problem.
+
+`git annex upgrade` to v6 configures a filter using `git-annex smudge`. With it, `git diff` can't be run on files external to the repository, without getting a lot of stderr output like:
+
+    fatal: '../test.txt' is outside repository
+    error: external filter 'git-annex smudge --clean %f' failed 1
+
+This breaks vim-gitgutter signs, and possibly other tools as well.
+
+
+### What steps will reproduce the problem?
+
+[[!format sh """
+$ git init test
+$ cd test
+$ git annex init
+$ git annex upgrade
+$ 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.