Introduce GH_TUPLE.

GH_TUPLE allows one to put all the GH_{ACCOUNT,PROJECT,TAGNAME} into one
variable, in the form of account:project:tagname[:group]. It is helpful
when there are many submodules.

PR: 204772
With hat: portmgr
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D4514