strmap: enable faster clearing and reusing of strmaps

When strmaps are used heavily, such as is done by my new merge-ort
algorithm, and strmaps need to be cleared but then re-used (because of
e.g. picking multiple commits to cherry-pick, or due to a recursive
merge having several different merges while recursing), free-ing and
reallocating map->table repeatedly can add up in time, especially since
it will likely be reallocated to a much smaller size but the previous
merge provides a good guide to the right size to use for the next merge.

Introduce strmap_partial_clear() to take advantage of this type of
situation; it will act similar to strmap_clear() except that
map->table's entries are zeroed instead of map->table being free'd.
Making use of this function reduced the cost of
clear_or_reinit_internal_opts() by about 20% in mert-ort, and dropped
the overall runtime of my rebase testcase by just under 2%.

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

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