fix: force update branch rather than closing and reopening PRs

[bcoe]

Now looks for an existing PR with the same ref before creating a new PR.

fixes: #141, #128

[codecov]

Codecov Report

Merging #152 into master will decrease coverage by 0.49%.
The diff coverage is 15.58%.

@@            Coverage Diff            @@
##           master     #152     +/-   ##
=========================================
- Coverage   53.11%   52.61%   -0.5%     
=========================================
  Files           9        9             
  Lines        1301     1338     +37     
  Branches       72       77      +5     
=========================================
+ Hits          691      704     +13     
- Misses        608      631     +23     
- Partials        2        3      +1

Impacted Files	Coverage Δ
src/release-pr.ts	34.24% <0%> (-0.16%)	⬇️
src/github.ts	46.47% <16%> (-0.81%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e887626...7ff4991. Read the comment docs.