create non-standard tag
git-svn: fix occasional "Failed to strip path" error on fetch next commit, try #2

When --stdlayout and --preserve-empty-dirs flags are used and a
directory becomes empty, two things happen:

Sometimes find_empty_directories() returns empty list and no empty dir
placeholder file created. This happens, because find_empty_directories()
marks all directories as non-empty, if at least one updated directory is
non-empty.

Sometimes git-svn dies with "Failed to strip path" error. This happens,
because find_empty_directories() returns git paths and
add_placeholder_file() expects svn paths
git-svn: fix empty dir tracking in non-standard branches