dim: Clarify which working tree is not clean

We call assert_repo_clean() from a few places and for some of them, it's
not clear PWD would matter, example: dim ub

Clarify what's the working tree dim is complaining about.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
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