For back-compat, if we find both an old and new env var for the same
flag, prefer the old. This matters because the docker image sets
GITSYNC_ROOT but some users still set GIT_SYNC_ROOT.
Env-flags are "flags" that can only be set by env var (see caveat below).
All of the real flags have a corresponding env-flag (kind of, but not
really). The real goal was to deprecate `--password` but keep the env
var as a documented interface.
This does that (though --password still works) and updates the usage and
manual.
This allows some future work to follow the pattern. We do not register
every CLI flag as an env-flag because the help text would be
duplicative. This probably wants a wrapper API that allows declaring of
abstract flags, with CLI, env, or both sources.
Caveat:
ACTUALLY, these still have a flag, but the flag is specially named and
hidden. This makes testing a little easier where passing flags is
handled well but env vars is not.
Tim Hockin