tmt lint fails with fmf path not in root

[LecrisUT]

Here is a minimum example:

$ tree -a
.
└── a
    ├── .fmf
    │   └── version
    └── plan.fmf
$ tmt --root a lint
/plan
pass correct attributes are used
fail No tree found in repo 'None', missing an '.fmf' directory?

with plan.fmf:

summary:
  simple example
discover:
  how: fmf
  filter: "tag: something"
execute:
  how: tmt

tmt --root a run works fine, but I did not confirm that others work.

[happz]

Will be fixed by #1851