In order to create diffs that work with the perforce server, when
posting from a git-p4 repo, we have to in effect linewise process the
diff output from git diff. For very large files this can take quite a
while and users can be confused as to what is going on.

The solution is to print something to the standard output when diffing a
very large file and update it with a bit of progress every 1000 lines.

in a git repo with extremely large text files, rbt post. See that the output 'Creating diff for large file...' is printed and a little spinner animation updated every 1000 lines.