Add fallback for getPrPatch.ts

[syossan27]

Hi, @ArtiomTr

This PR fixes #430

This change is a workaround for the limitation on diffs in Pull Requests , which is also mentioned in the ISSUE.

I adopted the method of using octokit.rest.repos.compareCommits to get the differences.
We had a similar situation with the "reviewdog", and I used it as a reference for this implementation.
ref: reviewdog/reviewdog#1697

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	94.04% (-0.23% 🔻)	773/822
🟢	Branches	86.76%	308/355
🟢	Functions	92.27%	167/181
🟢	Lines	93.5% (-0.25% 🔻)	690/738

Show files with reduced coverage 🔻

St.❔	File	Statements	Branches	Functions	Lines
🔴	utils/getPrPatch.ts	28.57% (-11.43% 🔻)	100%	0%	28.57% (-11.43% 🔻)

Test suite run success

166 tests passing in 55 suites.

Report generated by 🧪jest coverage report action from 7f750dd

[ArtiomTr]

Hello @syossan27 👋,

Thank you for the contribution.

@all-contributors please add @syossan27 for code

[ArtiomTr]

Your changes are available under v2.3.1 release