describe: Store commit_names in a hash table by commit SHA1

describe: Store commit_names in a hash table by commit SHA1

describe is currently forced to look up the commit at each tag in
order to store the struct commit_name pointers in struct commit.util.
For --exact-match queries, those lookups are wasteful.  In preparation
for removing them, put the commit_names into a hash table, indexed by
commit SHA1, that can be used to quickly check for exact matches.

Signed-off-by: Anders Kaseorg <andersk@ksplice.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>