revlog: adjust rewrite_sidedata code to not delete existing revlog content

The "w+" file mode is deleting all the content of the opened file. Which is bad…

This is not caught by the test because the test only check for a full, initial
pull where not pre-existing content exists. So we need to fix our test coverage
here.

However they are another issue that prevent "incremental" pull to work here. See
next changeset for details.

Differential Revision: https://phab.mercurial-scm.org/D10543

mercurial/revlog.py

@@ -3118,7 +3118,7 b' class revlog(object):'
                 current_offset += len(serialized_sidedata)
 
         # rewrite the new index entries
-        with self._indexfp(b'w+') as fp:
+        with self._indexfp(b'r+') as fp:
             fp.seek(startrev * self.index.entry_size)
             for i, e in enumerate(new_entries):
                 rev = startrev + i