Merge branch 'jk/t5551-expensive-test-timeouts-fix'

The Apache timeout in HTTP tests has been increased to prevent test
failures on heavily loaded CI runners. The tests creating an
enormous number of refs have been isolated to their own repositories
to avoid slowing down subsequent tests.

* jk/t5551-expensive-test-timeouts-fix:
  t5551: put many-tags case into its own repo
  t/lib-httpd: bump apache timeout