R1:c04147328fa3

Fix isValidGitShallowCloneResponse

Summary:
Changes the heuristic method by which non-zero exit statuses from git-http-backend are found to be due to packfile negotiation during shallow fetches, etc.

Instead of checking git-http-backend stderr for a generic "hung up" error message, see if the pack-result response contains a terminating flush packet ("0000"). This should give a greater assurance that the request was handled correctly and the response is complete.

Test Plan: Run `GIT_CURL_VERBOSE=1 git…