use a hashmap to make remotes fasterpr/remotes-in-hashmap

Remotes are stored as an array, so looking one up or adding one without
duplication is an O(n) operation.  Reading an entire config file full of
remotes is O(n^2) in the number of remotes.  For a repository with tens of
thousands of remotes, the running time can hit multiple minutes.

Hash tables are way faster.  So we add a hashmap from remote name to
struct remote and use it for all lookups.  The time to add a new remote to
a repo that already has 50,000 remotes drops from ~2 minutes to < 1
second.

We retain the old array of remotes so iterators proceed in config-file
order.

Signed-off-by: Patrick Reynolds <patrick.reynolds@github.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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