git-svn: fix broken symlink workaround when switching branches

git-svn: fix broken symlink workaround when switching branches

Thanks to Anton Gyllenberg <anton@iki.fi> for the bug report
(and testcase in the following commit):
> Commit dbc6c74d0858d77e61e092a48d467e725211f8e9 "git-svn:
> handle empty files marked as symlinks in SVN" caused a
> regression in an unusual case where a branch has been created
> in SVN, later deleted and then created again from another
> branch point and the original branch point had empty files not
> in the new branch. In some cases git svn fetch will then fail
> while trying to fetch the empty file from the wrong SVN
> revision.

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