{reset,merge}: call discard_index() before returning

{reset,merge}: call discard_index() before returning

These two built-ins both deal with the index, but weren't discarding
it. In subsequent commits we'll add more free()-ing to discard_index()
that we've missed, but let's first call the existing function.

We can doubtless add discard_index() (or its alias discard_cache()) to
a lot more places, but let's just add it here for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Taylor Blau <redacted>