unpack-trees: add 'skip_cache_tree_update' option

unpack-trees: add 'skip_cache_tree_update' option

Add (disabled by default) option to skip the 'cache_tree_update()' at the
end of 'unpack_trees()'. In many cases, this cache tree update is redundant
because the caller of 'unpack_trees()' immediately follows it with
'prime_cache_tree()', rebuilding the entire cache tree from scratch. While
these operations aren't the most expensive part of operations like 'git
reset', the duplicate calls still create a minor unnecessary slowdown.

Introduce an option for callers to skip the 'cache_tree_update()' in
'unpack_trees()' if it is redundant (that is, if 'prime_cache_tree()' is
called afterwards). At the moment, no 'unpack_trees()' callers use the new
option; they will be updated in subsequent patches.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>