Update the diff script to be smarter and diff against the merge base.

[nsthorat]

NOTE: do not update this PR's branch with master.

The diff script now clones and the current branch and commit as well as master rewinded to the merge base of the current branch and master to only find changes against where the branch was created (only true for branches on the main repo).

For forks, we diff against tensorflow/tfjs:master.

Test PRs:

• From a fork:master => DONT REVIEW: diff tool merge over test #1993
• From a fork:branch => DONT REVIEW: diff tool merge over test from nsthorat/tfjs:branch #2005
• This PR: from a branch that is behind master.

Note that this PR is behind tfjs-node version bump from 1.2.8 => 1.2.9

This change is 

[dsmilkov]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov and @nsthorat)

---

scripts/diff.js, line 44 at r1 (raw file):

  branchName = exec(`git rev-parse --abbrev-ref HEAD`).stdout.trim();
}
console.log('commitSha: ', commitSha);

are the log statements useful (actionable) to keep?

---

scripts/diff.js, line 67 at r1 (raw file):

  exec(`git fetch origin ${mergeBase}`);
  exec(`git checkout ${mergeBase}`);
  console.log('mergeBase: ', mergeBase);

useful to keep the log?

[nsthorat]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @dsmilkov)

---

scripts/diff.js, line 44 at r1 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

are the log statements useful (actionable) to keep?

spoke offline -- going to keep these because if the script goes wrong these are useful for debugging

---

scripts/diff.js, line 67 at r1 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

useful to keep the log?

see above