When allocating new tree iterator frames, we zero out the allocated
memory twice. Remove one of the `memset` calls.

When allocating tree iterator entries, we use GIT_ERROR_ALLOC_CHECK` to
check whether the allocation has failed. The macro will cause the
function to immediately return, though, leaving behind a partially
initialized iterator frame.

Fix the issue by manually checking for memory allocation errors and
using `goto done` in case of an error, popping the iterator frame.

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.

The filesystem iterator takes `stat` data from disk and puts them into
index entries, which use 32 bit ints for time (the seconds portion) and
filesize.  However, on most systems these are not 32 bit, thus will
typically invoke a warning.

Most users ignore these fields entirely.  Diff and checkout code do use
the values, however only for the cache to determine if they should check
file modification.  Thus, this is not a critical error (and will cause a
hash recomputation at worst).

Move to the `git_error` name in the internal API for error-related
functions.

Use the new object_type enumeration names within the codebase.

The function `tree_iterator_entry_cmp` has been introduced in commit be30387e8
(iterators: refactored tree iterator, 2016-02-25), but in fact it has never been
used at all. Remove it to avoid unused function warnings as soon as we re-enable
"-Wunused-functions".

Introduce a `git_iterator_foreach` helper function which invokes a
callback on all files for a given iterator.

Optionally hash the contents of files encountered in the filesystem or
working directory iterators.  This is not expected to be used in
production code paths, but may allow us to simplify some test contexts.

For working directory iterators, apply filters as appropriate, since we
have the context able to do it.