svn 1.6.x (at least, 1.6.12 or 1.6.17) might display empty lines, even
though svn 1.9.x (at least, 1.9.3) doesn't.
To make tests for convert with svn portable, this patch adds "|(^$)"
regexp to egrep in filter_svn_output.
To avoid similar future issue, this patch adds "|(^$)" regexp to all
filter_svn_output (and adjusts test-subrepo-svn.t), even though only
test-convert-svn-source.t fails with svn 1.6.x, AFAIK.
The subversion tests used different tricks to create properly encoded URLs,
partly due to partial support for different ways of running the tests on
windows. Now we only need/support one way of running the tests on windows.
Windows URLs should look like 'file:///c:/foo%20bar' and on Unix platforms
like 'file:///tmp/baz'.
'pwd' in the test framework will on Windows emit paths like 'c:/foo bar'.
Explicit handling of backslashes in paths is thus no longer needed and is
removed. Paths on windows do however need an extra '/' compared to other
platforms.
This change makes test-subrepo-svn.t pass on windows with msys. Other tests
might need more work.