gitserver: Lift best effort filepath.Walk pattern into helper

Created by: keegancsmith

This is to make it clearer our intentions in everyplace we use this pattern in gitserver.

Follow-up from https://github.com/sourcegraph/sourcegraph/pull/7030

Created by: codecov[bot]

Codecov Report

Merging #7032 into master will increase coverage by <.01%. The diff coverage is 90%.

@@            Coverage Diff             @@
##           master    #7032      +/-   ##
==========================================
+ Coverage   39.23%   39.23%   +<.01%     
==========================================
  Files        1230     1230              
  Lines       63535    63530       -5     
  Branches     6041     6041              
==========================================
- Hits        24926    24925       -1     
+ Misses      36326    36324       -2     
+ Partials     2283     2281       -2

Impacted Files	Coverage Δ
cmd/gitserver/server/serverutil.go	59.06% <100%> (+0.97%)
cmd/gitserver/server/cleanup.go	64.89% <80%> (+0.71%)
cmd/frontend/graphqlbackend/repository.go	20.73% <0%> (-0.93%)