Fix a problem with numbered environment variables

[jespino]

I found this problem here: go-task/task#586

I traced it to here. For some reason, the sorting + the binary search, were
behaving wrong. The inclusion of the = sign in the ordering looks like fixes
the problem. I'm not 100% sure why, maybe is again, an upstream bug.

I added a unit test to reproduce the problem.

[mvdan]

Oh wow, very nice catch! We rely on the list to be sorted for the binary search, and the binary search uses the = suffix to avoid the issue of "duplicate prefixes" like your GREETING and GREETING2. With the equals, GREETING= will never match the suffix of GREETING2=.

However, I forgot to also include the = suffix when sorting. So the two sort functions disagreed on the order of the items in this edge case.

[mvdan]

I'll merge on green, using a squash-merge to include the explanation of the fix in the commit message :)