SubversionDumper fixes

This merge request provides various improvements to the Subversion dumper code to fix issues encountered when converting some simple git repositories to svn. Additionally I have run all the test fast-import streams (i.e. test/*.fi) and test svn dumps (i.e. test/*svn) through reposurgeon (reposurgeon "read -" "prefer svn" "write -" < in > out) and in all cases was able to successfully import the resulting dump file into Subversion. Looking at the diff between the input and output svn dumps the major changes are the branch structure is standardised, creating a branch/tag from a subdirectory does not work cleanly (see agito.svn as an example of this issue) and symlinks are not preserved.

No tests are added as part of this request but they would be desirable (maybe using the existing test inputs). These changes should also help in understanding the remaining deficiencies in the Subversion parser.

Replaces: !65 (closed) and !67 (closed) (reworded commit messages and added fixes from test inputs). Fixes: #70 (closed) and #71 (closed)