Compare against the merge-base if no vs is specified

[maleadt]

Currently, PkgEvalJob was comparing against the git ref as specified (which is bad because you could then be comparing against a branch that's much ahead, e.g., JuliaLang/julia#47369 (comment)), and BenchmarkJob was checking out the merge commit. I'd argue that the latter isn't ideal as well: sometimes the merge commit is unavailable (in case of merge conflicts) leading to inconsistent behavior; the merge commit isn't available directly so if you request runbenchmarks as part of the PR opening post you'll end up comparing against a different commit; and finally the merge base is a commit that you can't find in the GitHub UI, so the versioninfo() as reported in the benchmark reports isn't very useful.

As an alternative, this PR switches to using the merge-base, i.e., the commit on master where the PR forked. However, as it's not clear when to look up the merge-base (e.g., only when specifying vs=:master, or also when specifying vs=v1.8 or vs=SOME_SHA), I've implemented @KristofferC's suggestion here to default to the master branch when not specifying a vs at all from a Nanosoldier invocation on a PR. This also considerably simplifies the most common Nanosoldier invocation to: @nanosoldier runbenchmarks(ALL) or @nanosoldier runtests(ALL).

I'll be testing this on the PkgEval bot in the coming days.

Fixes #131

[maleadt]

As suggested by @KristofferC: maybe ALL should be the default package selection so that in the common case you just do runtests().

[maleadt]

@vtjnash Any thoughts, as this does change BenchmarkJob?

[vtjnash]

That rationale all seems reasonable