sha1_file: drop experimental GIT_USE_LOOKUP search

sha1_file: drop experimental GIT_USE_LOOKUP search

Long ago in 628522ec14 (sha1-lookup: more memory efficient
search in sorted list of SHA-1, 2007-12-29) we added
sha1_entry_pos(), a binary search that uses the uniform
distribution of sha1s to scale the selection of mid-points.
As this was a performance experiment, we tied it to the
GIT_USE_LOOKUP environment variable and never enabled it by
default.

This code was successful in reducing the number of steps in
each search. But the overhead of the scaling ends up making
it slower when the cache is warm. Here are best-of-five
timings for running rev-list on linux.git, which will have
to look up every object:

  $ time git rev-list --objects --all >/dev/null
  real	0m35.357s
  user	0m35.016s
  sys	0m0.340s

  $ time GIT_USE_LOOKUP=1 git rev-list --objects --all >/dev/null
  real	0m37.364s
  user	0m37.045s
  sys	0m0.316s

The USE_LOOKUP version might have more benefit on a cold
cache, as the time to fault in each page would dominate. But
that would be for a single lookup. In practice, most
operations tend to look up many objects, and the whole pack
.idx will end up warm.

It's possible that the code could be better optimized to
compete with a naive binary search for the warm-cache case,
and we could have the best of both worlds. But over the
years nobody has done so, and this is largely dead code that
is rarely run outside of the test suite. Let's drop it in
the name of simplicity.

This lets us remove sha1_entry_pos() entirely, as the .idx
lookup code was the only caller.  Note that sha1-lookup.c
still contains sha1_pos(), which differs from
sha1_entry_pos() in two ways:

  - it has a different interface; it uses a function pointer
    to access sha1 entries rather than a size/offset pair
    describing the table's memory layout

  - it only scales the initial selection of "mi", rather
    than each iteration of the search

We can't get rid of this function, as it's called from
several places. It may be that we could replace it with a
simple binary search, but that's out of scope for this patch
(and would need benchmarking).

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