perf: cache .git/index for better performance

[billiegoose]

I'm improving performance

This speeds up running statusMatrix on large repos like https://github.com/ianstormtaylor/slate from ~80 seconds to ~1 second. (These times measured on a proprietary Electron app. How big the perf hit is depends highly on the latency of the filesystem... on Electron the latency is very bad because it is proxied to the worker over IPC.)

Possible Dangers

It does caching based on the compound key [fs, filepathOfIndexFile]. It uses lstat to determine whether the index file has been modified. 99% of the time this should be enough! However... if you are "resetting" a filesystem mock behind the scenes (like my BrowserFS tests were), that might not be enough. In such cases that the index file returns the same stat information but it actually has changed different, then your tests may too fail. (This caching may also be susceptible to "racy-git"-like race conditions, since it depends on the last-modified timestamp.)

The solution is to make sure whenever you are "resetting" the filesystem to set the filesystem to a new object, i.e.

plugins.set('fs', {...fs})

so that the old cached index file will not be used.

[isomorphic-git-bot]

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

[karma-pr-reporter]

Test Results for 563b3e7:

Browser	Passed	Skipped	Failed	Time	Disconnected	Error
HeadlessChrome 0.0.0 (Linux 0.0.0)	265	1	0	1 min 19.605 secs	false	undefined
Mobile Safari 11.0.0 (iOS 11.2.0)	81	0	0	2 mins 5.011 secs	false	undefined
Mobile Safari 11.0.0 (iOS 11.2.0)	81	0	0	2 mins 5.043 secs	false	undefined
Firefox 67.0.0 (Ubuntu 0.0.0)	265	1	0	2 mins 6.456 secs	false	undefined
Edge 17.17134.0 (Windows 10 0.0.0)	81	0	0	2 mins 23.464 secs	false	undefined
Edge 17.17134.0 (Windows 10 0.0.0)	81	0	0	2 mins 23.479 secs	false	undefined
Safari 11.1.2 (Mac OS X 10.13.6)	265	1	0	3 mins 40.967 secs	false	undefined
Chrome Mobile 67.0.3396 (Android 7.1.1)	264	1	1	4 mins 6.131 secs	false	undefined

[karma-pr-reporter]

Test Results for 563b3e7:

Browser	Passed	Skipped	Failed	Time	Disconnected	Error
Mobile Safari 11.0.0 (iOS 11.2.0)	81	0	0	2 mins 13.696 secs	false	undefined
Mobile Safari 11.0.0 (iOS 11.2.0)	81	0	0	2 mins 13.718 secs	false	undefined
Edge 17.17134.0 (Windows 10 0.0.0)	81	0	0	2 mins 16.07 secs	false	undefined
Edge 17.17134.0 (Windows 10 0.0.0)	81	0	0	2 mins 16.078 secs	false	undefined
Chrome Mobile 67.0.3396 (Android 7.1.1)	264	1	1	3 mins 43.29 secs	false	undefined

[karma-pr-reporter]

Test Results for e365122:

Browser	Passed	Skipped	Failed	Time	Disconnected	Error
HeadlessChrome 0.0.0 (Linux 0.0.0)	265	1	0	1 min 16.391 secs	false	undefined
Firefox 67.0.0 (Ubuntu 0.0.0)	265	1	0	2 mins 0.016 secs	false	undefined
Edge 17.17134.0 (Windows 10 0.0.0)	265	1	0	5 mins 17.983 secs	false	undefined

[karma-pr-reporter]

Test Results for e365122:

Browser	Passed	Skipped	Failed	Time	Disconnected	Error
Chrome Mobile 67.0.3396 (Android 7.1.1)	265	1	0	3 mins 17.253 secs	false	undefined
Safari 11.1.2 (Mac OS X 10.13.6)	265	1	0	3 mins 51.82 secs	false	undefined
Mobile Safari 11.0.0 (iOS 11.2.0)	265	1	0	4 mins 29.894 secs	false	undefined

[karma-pr-reporter]

Test Results for a3d50e7:

Browser	Passed	Skipped	Failed	Time	Disconnected	Error
HeadlessChrome 0.0.0 (Linux 0.0.0)	265	1	0	1 min 19.512 secs	false	undefined
Firefox 67.0.0 (Ubuntu 0.0.0)	265	1	0	2 mins 11.537 secs	false	undefined
Safari 11.1.2 (Mac OS X 10.13.6)	265	1	0	2 mins 45.956 secs	false	undefined
Chrome Mobile 67.0.3396 (Android 7.1.1)	265	1	0	3 mins 51.553 secs	false	undefined
Mobile Safari 11.0.0 (iOS 11.2.0)	265	1	0	4 mins 21.484 secs	false	undefined
Edge 17.17134.0 (Windows 10 0.0.0)	265	1	0	5 mins 30.32 secs	false	undefined

[isomorphic-git-bot]

🎉 This PR is included in version 0.58.1 🎉

The release is available on:

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

Your semantic-release bot 📦🚀