dir.c: free strings in sparse cone pattern hashmaps

dir.c: free strings in sparse cone pattern hashmaps

The pattern_list structs used for cone-mode sparse lookups use a few
extra hashmaps. These store pattern_entry structs, each of which has its
own heap-allocated pattern string. When we clean up the hashmaps, we
free the individual pattern_entry structs, but forget to clean up the
embedded strings, causing memory leaks.

We can fix this by iterating over the hashmaps to free the extra
strings. This reduces the numbers of leaks in t7002 from 22 to 9.

One alternative here would be to make the string a FLEX_ARRAY member of
the pattern_entry. Then there's no extra free() required, and as a bonus
it would be a little more efficient. However, some of the refactoring
gets awkward, as we are often assigning strings allocated by helper
functions. So let's just fix the leak for now, and we can explore bigger
refactoring separately.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>