vcs-svn: pass paths through to fast-import

vcs-svn: pass paths through to fast-import

Now that there is no internal representation of the repo, it is not
necessary to tokenise paths.  Use strbuf instead and bypass
string_pool.

This means svn-fe can handle arbitrarily long paths (as long as a
strbuf can fit them), with arbitrarily many path components.

While at it, since we now treat paths in their entirety, only quote
when necessary.

Signed-off-by: David Barr <redacted>
Signed-off-by: Jonathan Nieder <redacted>