fix: optimize speed of retrieving user/group widths

[owallb]

After using the new user and group info parameters for a while I have started to notice an occasional slow-down, when quickly navigating down/up in the same directory.

I investigated the time it would take retrieve user and group max widths, where total is the whole getInfo loop:

Unoptimized, average over N=58, time in ms:	getUserWidth	getGroupWidth	Total
0.01	15.59	15.62

Optimized, average over N=80, time in ms:	getUserGroup	Total
0.01	0.03

I'm not really sure why getGroupWidth was so slow.

15 ms is not slow enough that I would notice it, what I experienced was that the UI would freeze for about 1 second. But it's probably related to the cause of it.

[owallb]

I made a mistake and didn't call groupName for retrieving the group name. Now that I fixed it, the original issue still remains, which is that calling groupName will sometimes be very slow. I will close this and continue to investigate a different solution.

[owallb]

For future reference, the issue was that I had built with CGO_ENABLED=1. Instead building with CGO_ENABLED=0 fixed the random freezes. It seems the cgo implementation of LookupGroupId in os/user is slow for whatever reason.