gnu: services: Add cgit.

* gnu/services/version-control.scm
(<cgit-configuration-file>, <cgit-configuration>): New record types.
(cgit-configuration-robots-string, cgit-activation,
cgit-configuration-nginx-config): New procedures.
(%cgit-configuration-nginx, cgit-service-type): New variables.
* gnu/tests/version-control.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (Version Control): Document the cgit service.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>