Fix commit-comment benchmark comparison

[samuelburnham]

Alternate idea to #861: trades a bit of parsing for getting the PR number in the commit-comment, which might be nice for easy lookup.

Note: Because criterion-table parses by /, this solution will likely be buggy when attempting to merge into a non-default base branch with a / character (e.g. sb/base-branch will be truncated by criterion-table into fib-branch=sb).

[huitseeker]

The VERGEN_GIT_BRANCH has content that's :

• resolved to gh-merge-queue in merge_group,
• likely to shift / not lead to reproducible builds, and I expect folks to demand precise pointers to code.

Same goes for the PR number.