patch: make diff --git to differentiate b/w file is empty or doesn't exists Before this patch, as we didn't differentiate the two cases of a file in a context: 1. File doesn't exists 2. File is empty which causes the blob id to be same for both the cases. Now we use `nullhex` for a file which doesn't exists in a context (aligning it with the git diff format) Changes in test file reflect the fixed behavior. Differential Revision: https://phab.mercurial-scm.org/D10001