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-test: don't test for possible bug "Failed to strip path", try #2

In the previous commit a showcase was added to
t9160-git-svn-preserve-empty-dirs.sh for the "Failed to strip path" bug.
Now the flag --stdlayout should be enough.
git-svn: fix svn fetch erroneously recreating empty dir placeholder deleted earlier, try #2

The Fetcher accumulates deleted paths in an array and doesn't reset the
array on next commit. This causes different results when interrupting
and resuming the fetch.
When --preserve-empty-dirs flag is used, a path in the array can be
erroneously treated as just deleted (although it was deleted in the
previous commit) and cause the creation of an empty dir placeholder.