Augment revisionid-to-diffid.sh to work with github. (#1)
Augment revisionid-to-diffid.sh to work with github. (#1)

## Summary:
Now you can say
```
    revisionid-to-diffid.sh #1710
```
and it will resolve to what branch that is.  This means that
`git co #1710` (or alternately `git co 1710`) can be used as
a way to change branches.

Needs `gh` to be installed.

As part of this, I got rid of the need for the `--tag` argument.  I
grepped through a bunch of repos, and we never use
revisionid-to-diffid.sh without `--tag`.

Issue: https://khanacademy.slack.com/archives/C013ANU53LK/p1634071670322800

## Test plan:
I ran these manually:
```
%  revisionid-to-diffid.sh 1768
forward-browser-headers

% revisionid-to-diffid.sh #1768
forward-browser-headers

% revisionid-to-diffid.sh D1234
fatal: couldn't find remote ref refs/tags/phabricator/diff/1107
WARNING: unable to fetch 'phabricator/diff/1107'; perhaps it was never pushed?
phabricator/diff/1107

% revisionid-to-diffid.sh 1
structured-logging

% revisionid-to-diffid.sh #1
structured-logging

% revisionid-to-diffid.sh ##1
no pull requests found for branch "##1"

% revisionid-to-diffid.sh csilvers
csilvers
```

Author: csilvers

Reviewers: lillialexis, dbraley, jaredly

Required Reviewers:

Approved By: lillialexis

Checks:

Pull Request URL: https://github.com/Khan/git-workflow/pull/1
Khan/git-workflow,Khan/git-workflow