fix: snapshots will now be taken from the latest commit

[bcoe]

snapshot releases were pinning at ancient SHAs in some cases, because we use the last tag rather than the latest commit at head.

We now use the latest commit at head.

[codecov]

Codecov Report

Merging #400 into master will decrease coverage by 1.77%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
- Coverage   89.08%   87.31%   -1.78%     
==========================================
  Files          27       27              
  Lines        3170     3176       +6     
  Branches      296      240      -56     
==========================================
- Hits         2824     2773      -51     
- Misses        346      400      +54     
- Partials        0        3       +3     

Impacted Files	Coverage Δ
src/releasers/java-yoshi.ts	91.13% <88.88%> (-2.02%)	⬇️
src/release-pr.ts	85.77% <0.00%> (-4.35%)	⬇️
src/github.ts	72.37% <0.00%> (-2.82%)	⬇️
src/github-release.ts	95.20% <0.00%> (-2.40%)	⬇️
src/releasers/php-yoshi.ts	91.01% <0.00%> (-2.35%)	⬇️
src/graphql-to-commits.ts	88.46% <0.00%> (-1.65%)	⬇️
src/conventional-commits.ts	97.27% <0.00%> (-0.46%)	⬇️

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