merge-ort: fix accidental strset<->strintmap

merge-ort: fix accidental strset<->strintmap

Both strset_for_each_entry and strintmap_for_each_entry are macros that
evaluate to the same thing, so they are technically interchangeable.
However, the intent is that we use the one matching the variable type we
are passing.  Unfortunately, I somehow mistakenly got one of these wrong
in 7bee6c100431 (merge-ort: avoid recursing into directories when we
don't need to, 2021-07-16) -- possibly related to the fact that
relevant_sources was initially a strset and later refactored into a
strintmap.  Correct which macro we use.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>