describe: refresh the index when 'broken' flag is used

describe: refresh the index when 'broken' flag is used

When describe is run with 'dirty' flag, we refresh the index
to make sure it is in sync with the filesystem before
determining if the working tree is dirty.  However, this is
not done for the codepath where the 'broken' flag is used.

This causes `git describe --broken --dirty` to false
positively report the worktree being dirty if a file has
different stat info than what is recorded in the index.
Running `git update-index -q --refresh` to refresh the index
before running diff-index fixes the problem.

Also add tests to deliberately update stat info of a
file before running describe to verify it behaves correctly.

Reported-by: Paul Millar <paul.millar@desy.de>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Abhijeet Sonar <abhijeet.nkt@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>