gogs: Ensure to list all repositories

In contrary of gitea listing which does not require to provide the q query
parameter, it is required for the gogs case.

After reading the gogs source code, the /repos/search endpoint generates
a sql request of the form: "SELECT * FROM repos WHERE name LIKE '%{q}%'".
By setting the q parameter value to "_", the LIKE clause acts as a
wildcard and all repositories are ensured to be returned.

Fixes #4698.