git-svn: fix delete+add branch tracking with empty files

git-svn: fix delete+add branch tracking with empty files

Original bug report and test case by Björn Steinbrink.

Björn Steinbrink <B.Steinbrink@gmx.de> wrote:
> Hi Eric,
>
> seems that the empty symlink stuff gets confused about which revision to
> use when looking for the parent's file.
>
> r3 = f1a6fcf6b0a1c4a373d0b2b65a3d70700084f361 (tags/1.0.1)
> Found possible branch point: file:///home/doener/h/svn/tags/1.0 => file:///home/doener/h/svn/branches/1.0, 4
> Found branch parent: (1.0) 63ae640ba01014ecbb3df590999ed1fa5914545b
> Following parent with do_switch
> Successfully followed parent
> r5 = 26fcfef5bcced97ab74faf1af7341a2ae0d272aa (1.0)
> Found possible branch point: file:///home/doener/h/svn/branches/1.0 => file:///home/doener/h/svn/tags/1.0.1, 5
> Found branch parent: (tags/1.0.1) 26fcfef5bcced97ab74faf1af7341a2ae0d272aa
> Following parent with do_switch
> Scanning for empty symlinks, this may take a while if you have many empty files
> You may disable this with `git config svn.brokenSymlinkWorkaround false'.
> This may be done in a different terminal without restarting git svn
> Filesystem has no item: File not found: revision 3, path '/branches/1.0/file' at /usr/local/libexec/git-core/git-svn line 3318
>
> Note how it tries to look at revision 3 instead of revision 5 (which it
> correctly detected as the parent). The import succeeds when
> svn.brokenSymlinkWorkaround is set to false. Testcase below.

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