Fix commit ID detection on Bitbucket Server single file code views

Created by: lguychard

Fixes #4593

This unfortunately relies on inspecting the document's context (vs. just the code view context). It was the case before this fix as well (getCommitIDFromLink() uses document.querySelector()). The alternative would be to resolve single file code views to an element that encompasses the repo header:

Created by: codecov[bot]

Codecov Report

Merging #4807 into master will increase coverage by 0.01%. The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #4807      +/-   ##
==========================================
+ Coverage   48.11%   48.13%   +0.01%     
==========================================
  Files         726      726              
  Lines       44263    44269       +6     
  Branches     1761     1761              
==========================================
+ Hits        21299    21307       +8     
+ Misses      20983    20980       -3     
- Partials     1981     1982       +1

Impacted Files	Coverage Δ
browser/src/libs/bitbucket/scrape.ts	60.17% <71.42%> (+14.38%)
cmd/frontend/graphqlbackend/graphqlbackend.go	46.85% <0%> (-6.3%)
...d/frontend/internal/authz/bitbucketserver/store.go	78.69% <0%> (-1.19%)