Investigate a performant replacement for createPatch from diff library

[joyceerhl]

Loading a PR containing a very large file (34K LOC) with changes takes ~1 minute: microsoft/vscode-pull-request-github#3187

This is because GHPRI depends on RemoteHub for generating diffs, and RemoteHub uses the diff library, which suffers from known performance issues: kpdecker/jsdiff#239

GHPRI is mitigating this by not generating diffs until absolutely necessary, but a user can still get unlucky if the first file in the PR is very large and we auto-display the diff, or the user may have to wait a long time after clicking to view file changes in the diff view.

We should identify a more performant library, e.g. Google's diff-match-patch js implementation plus modifications to output unidiffs.

[joyceerhl]

@alexr00, could you try setting "remoteHub.diff.useDiffMatchPatch": true in your settings.json and letting me know how it works for you over the next few days? The original diff library RemoteHub uses suffers from performance issues because it generates enough garbage that the VM spends >60% of its time GC-ing--I'm still looking into why.

From my testing, with the new library, it looks like a 50% speedup when loading https://insiders.vscode.dev/github/python/cpython/pull/29581 even without your recent GHPRI changes.

I am not sure if it has perfect feature parity with the original library so I don't want to enable it more broadly yet.

[alexr00]

Cool, I will try it out!

[joyceerhl]

Looks like the new diff library is a big improvement perf-wise (still behind the remoteHub.diff.useDiffMatchPatch toggle). Here are some preliminary results from testing RemoteHub with GHPRI:

RemoteHub	GHPRI	Time to PR view	Recording
Stable	Stable	60s	Recording
Insiders using new diff library	Stable	22s	Recording
Insiders using old diff library	Insiders	15s	Recording
Insiders using new diff library	Insiders	14s	Recording

Stable GHPRI, stable RemoteHub using old diff library

stable.mp4

Stable GHPRI, Insiders RemoteHub using new diff library

stable-rh.mp4

Insiders GHPRI, Insiders RemoteHub using old diff library

old-diff.mp4

Insiders GHPRI, Insiders RemoteHub using new diff library

new-diff.mp4