rsync: use a more robust check for relative link than "startswith"

`startswith` is incorrect for paths in general, e.g. doesn't handle
Windows extended-length paths and other such.

The particular code here also wasn't careful about `/a/bc` not being
relative to `/a/b`.

Author: bluetech

execnet/rsync.py

@@ -173,10 +173,16 @@ def _send_directory(self, path):
             self._send_directory_structure(p)
 
     def _send_link_structure(self, path):
-        linkpoint = os.readlink(path)
         basename = path[len(self._sourcedir) + 1 :]
-        if linkpoint.startswith(self._sourcedir):
-            self._send_link("linkbase", basename, linkpoint[len(self._sourcedir) + 1 :])
+        linkpoint = os.readlink(path)
+        try:
+            relpath = os.path.relpath(linkpoint, self._sourcedir)
+        except ValueError:
+            relpath = None
+        if relpath not in (None, os.curdir, os.pardir) and not relpath.startswith(
+            os.pardir + os.sep
+        ):
+            self._send_link("linkbase", basename, relpath)
         else:
             # relative or absolute link, just send it
             self._send_link("link", basename, linkpoint)