Improved tree iterator internals

This updates the tree iterator internals to be more efficient.

The tree_iterator_entry objects are now kept as pointers that are
allocated from a git_pool, so that we may use git__tsort_r for
sorting (which is better than qsort, given that the tree is
likely mostly ordered already).

Those tree_iterator_entry objects now keep direct pointers to the
data they refer to instead of keeping indirect index values.  This
simplifies a lot of the data structure traversal code.

This also adds bsearch to find the start item position for range-
limited tree iterators, and is more explicit about using
git_path_cmp instead of reimplementing it.  The git_path_cmp
changed a bit to make it easier for tree_iterators to use it (but
it was barely being used previously, so not a big deal).

This adds a git_pool_free_array function that efficiently frees a
list of pool allocated pointers (which the tree_iterator keeps).
Also, added new tests for the git_pool free list functionality
that was not previously being tested (or used).
Fix valgrind issues (and mmap fallback for diff)

This fixes a number of issues identified by valgrind - mostly
missed free calls.  Inside valgrind, mmap() may fail which causes
some of the diff tests to fail.  This adds a fallback code path
to diff_output.c:get_workdir_content() where is the mmap() fails
the code will now try to read the file data directly into allocated
memory (which is what it would do if the data needed to be filtered
anyhow).
Added pool freelist struct for readability

This adds a git_pool_freelist_item struct that makes it a little
easier to follow what's going on with the pool free list block
management code.  It is functionally neutral.
Merge pull request #1413 from arrbee/more-iterator-refactor

Further tree_iterator refactoring