git-svn: workaround a for broken symlinks in SVN

git-svn: workaround a for broken symlinks in SVN

It's possible for bad clients to commit symlinks without the
5-character "link " prefix in symlinks.  So guard around this
bug in SVN and make a best effort to create symlinks if the
"link " prefix is missing.

More information on this SVN bug is described here:
  http://subversion.tigris.org/issues/show_bug.cgi?id=2692

To be pedantic, there is still a corner case that neither we nor
SVN can handle:  If somebody made a link using a broken SVN
client where "link " is the first part of its path, e.g.
"link sausage", then we'd end up having a symlink which points
to "sausage" because we incorrectly stripped the "link ".
Hopefully this hasn't happened in practice, but if it has,
it's not our fault SVN is broken :)

Thanks to Benoit Sigoure and Sverre Johansen for reporting
and feedback.

Signed-off-by: Eric Wong <normalperson@yhbt.net>