config: support "inboxdir" in addition to "mainrepo"
"mainrepo" ws a bad name and artifact from the early days when I
intended for there to be a "spamrepo" (now just the
ENV{PI_EMERGENCY} Maildir). With v2, "mainrepo" can be
especially confusing, since v2 needs at least two git
repositories (epoch + all.git) to function and we shouldn't
confuse users by having them point to a git repository for v2.
Much of our documentation already references "INBOX_DIR" for
command-line arguments, so use "inboxdir" as the
git-config(1)-friendly variant for that.
"mainrepo" remains supported indefinitely for compatibility.
Users may need to revert to old versions, or may be referring
to old documentation and must not be forced to change config
files to account for this change.
So if you're using "mainrepo" today, I do NOT recommend changing
it right away because other bugs can lurk.
Link: https://public-inbox.org/meta/874l0ice8v.fsf@alyssa.is/