gitserver: Avoid double HTTP response header writing

Created by: tsenart

Avoids errors like http: superfluous response.WriteHeader call from github.com/sourcegraph/sourcegraph/cmd/gitserver/server.(*gitServiceHandler).ServeHTTP (gitservice.go:108)

Created by: sourcegraph-bot

Notifying subscribers in CODENOTIFY files for diff 023542e396de2c81e8bbadfb6e0429cd4e77cbfc...5b7efdb9d20eedb6628a3934cac1bb63e0c16afd.

Notify	File(s)
@keegancsmith	cmd/gitserver/server/gitservice.go

Created by: codecov[bot]

Codecov Report

Merging #16072 (5b7efdb) into main (023542e) will decrease coverage by 0.00%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main   #16072      +/-   ##
==========================================
- Coverage   52.83%   52.83%   -0.01%     
==========================================
  Files        1641     1641              
  Lines       82248    82248              
  Branches     7337     7337              
==========================================
- Hits        43456    43455       -1     
- Misses      34943    34944       +1     
  Partials     3849     3849              

Flag	Coverage Δ		*Carryforward flag
go	52.66% <0.00%> (-0.01%)
integration	28.65% <ø> (ø)		Carriedforward from 023542e
storybook	27.89% <ø> (ø)		Carriedforward from 023542e
typescript	53.26% <ø> (ø)		Carriedforward from 023542e
unit	35.38% <ø> (ø)		Carriedforward from 023542e

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
cmd/gitserver/server/gitservice.go	75.92% <0.00%> (ø)
cmd/repo-updater/repos/types.go	74.20% <0.00%> (-0.25%)