Ignore local history when we miss an object

Summary:
The git protocol will not allow us to "fill the gaps" if we just filter out the
missing HEADs: if we have data that's been created more recently, the packfile
returned by the server will be empty.

Test Plan:
Loading an origin manually, then removing one of the HEADs, then
loading again fills the gap.

Reviewers: Reviewers!

Differential Revision: https://forge.softwareheritage.org/D456