name-hash: perf improvement for lazy_init_name_hash

Improve performance of lazy_init_name_hash() when
ignore_case is set.  Teach name-hash to build the
istate.name_hash and istate.dir_hash simultaneously
using a forward-diving technique on the pathname
of the index_entry, rather than adding name_hash
entries and then searching backwards in the pathname
for parent directories.

This borrows algorithm ideas from clear_ce_flags_{1,dir}.

Multiple threads are used with the new algorithm to
speed hashmap construction.

This new code path is only used when threads are present
(a compiler settings) and when the index is large enough
to warrant the pthread complexity.

The code in clear_ce_flags_dir() uses a linear search to
find the adjacent index entries with the same prefix; a
binary search is used here handle_range_dir() to further
speed things up.

The size of LAZY_THREAD_COST was determined from rough
analysis using:
    t/helper/test-lazy-init-name-hash --analyze

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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