perf(walker): speeds up walking git repos

[billiegoose]

I'm improving performance

without changing any existing behaviors AFAICT.

[isomorphic-git-bot]

Thank you wmhilton! I auto-formatted the code using prettier-standard. 🤖

[karma-pr-reporter]

Test Results for fd2c349:

Browser	Pass	Skip	Fail	Time	Disconnect?
Chrome Mobile 69.0.3497 (Android 7.1.1)	296	1	0	2m:55s	false
Edge 17.17134.0 (Windows 10 0.0.0)	296	1	0	4m:38s	false
Firefox 69.0.0 (Ubuntu 0.0.0)	296	1	0	2m:1s	false
HeadlessChrome 0.0.0 (Linux 0.0.0)	296	1	0	0m:37s	false
Mobile Safari 11.0.0 (iOS 11.2.0)	295	2	0	3m:28s	false
Safari 11.1.2 (Mac OS X 10.13.6)	296	1	0	3m:46s	false

Longest running tests

• 43.58 secs - fetch fetch --prune-tags from git-http-mock-server - Safari
• 29.287 secs - fetch fetch (from Github) - Edge
• 15.426 secs - merge merge 'delete-first-half' and 'delete-second-half' (dryRun, missing author) - Edge
• 13.875 secs - fetch fetch --prune-tags from git-http-mock-server - Firefox
• 13.689 secs - fetch shallow fetch (from Github) - Edge
• 13.515 secs - fetch shallow fetch relative (from Github) - Edge
• 10.176 secs - fetch fetch --prune-tags from git-http-mock-server - Chrome Mobile
• 9.988 secs - fetch fetch (from Github) - Firefox
• 9.573 secs - GitPackIndex to .idx file from .pack - Firefox
• 8.414 secs - GitPackIndex from .pack - Firefox
• 7.809 secs - fetch shallow fetch exclude (from Github) - Edge

[isomorphic-git-bot]

🎉 This PR is included in version 0.67.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[isomorphic-git-bot]

🎉 This PR is included in version 1.0.0-beta.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀