zoekt-indexserver: allow adding plain git repositories

[rossburton]

I have a customised cgit server that the cgit indexer doesn't work on, but also only have a relatively small number of repositories to scan.

I think it should be possible to do something like this in the config JSON:

[
{ "GitURL": "http://git.server/repo-one", "Name": foo/repo-one"},
{ "GitURL": "http://git.server/repo-two", "Name": foo/repo-two"}
]

[linear]

SPLF-1455

[keegancsmith]

Welcome to accept a contribution adding this.