Fix AWS CodeCommit error MaximumRepositoryNamesExceededException

[nicksnyder]

fixes https://github.com/sourcegraph/sourcegraph/issues/968

[codecov-io]

Codecov Report

Merging #1023 into master will increase coverage by 0.08%.
The diff coverage is 0%.

Impacted Files	Coverage Δ
cmd/repo-updater/repos/awscodecommit.go	3.26% <0%> (+0.01%)	⬆️
.../internal/registry/extension_connection_graphql.go	26.53% <0%> (-0.56%)	⬇️
...md/frontend/internal/registry/extension_graphql.go	2.94% <0%> (-0.19%)	⬇️
cmd/frontend/registry/extension_remote_graphql.go	0% <0%> (ø)	⬆️
enterprise/cmd/xlang-go/internal/server/deps.go	81.62% <0%> (+0.85%)	⬆️
.../frontend/registry/extension_connection_graphql.go	1.19% <0%> (+1.19%)	⬆️
...se/cmd/frontend/internal/registry/extensions_db.go	70.98% <0%> (+3.17%)	⬆️
cmd/frontend/registry/extensions.go	53.84% <0%> (+4.31%)	⬆️
cmd/frontend/registry/extension_manifest.go	51.61% <0%> (+51.61%)	⬆️