opam config subst does not support absolute pathnames

[kit-ty-kate]

$ opam config subst /tmp/somefile
Fatal error:
OpamSystem.File_not_found("$PWD/tmp/somefile.in")

Detected while reviewing #6910

[NathanReb]

opam config subst is a bit more broken than that: opam config subst some/relative/path will try to subst some/relative/path.in.

[NathanReb]

I'm assuming it should rewrite the file in place, is that correct?

[kit-ty-kate]

it is what the man page suggests yeah, however i'm not sure if it is what we want and it would be a complete change of behaviour from what was there for the past 14 years (ced707e). But we could think about it in a separate ticket, although i'm not exactly sure how used/useful that command is