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
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