Hi there,

I noticed while debugging an issue in Phabricator where shallow git
fetches over HTTP were failing with 500 errors that `git-http-backend`
seemed to exit abnormally after the first request even though the
response was otherwise correct—and when the error is not surfaced to
the client, the packfile negotiation via `git-remote-curl` seems to
continue normally.[1]

I think this patch fixes the issue by having `upload-pack` gently
handle EOFs at a specific point in negotiation—after shallow/unshallow
lines have been returned to the client (followed by flush) but before
the client sends its haves.

This is my first contribution here so hopefully I'm understanding the
packfile negotiation protocol correctly and included my test in the
right place.

Kindly,
Daniel

[1] https://discourse.phabricator-community.org/t/git-fetches-with-depth-over-http-results-in-500-errors/4317