When revision is "latest", get the source code from the revision of the last successful coverage run rather than "tip"

[gijsk]

(potentially related: #524 )

When looking at https://coverage.moz.tools/#revision=latest&path=browser%2Fbase%2Fcontent%2Fbrowser.js&view=file&line=7938 (actually around line 7938!) there are some commented lines that are apparently "covered", some code lines that are "non-instrumented", and some method signatures labeled "uncovered" while their contents are "covered".

This makes me suspect the line numbers in the coverage data are off somehow.

[gijsk]

@marco-c points out that this used to be correct: https://coverage.moz.tools/#revision=d24d00b719aa&path=browser%2Fbase%2Fcontent%2Fbrowser.js&view=file shows reasonable data.

[marco-c]

The issue was that for a couple of days the coverage generation was broken, and when using revision=latest we are getting the code from "tip" but the coverage from the latest successful coverage run.

[marco-c]

The pressing issue is now fixed, so the coverage no longer looks weird.
It's still worth fixing this though.