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.

This should have been part of PR #3638. Without this we still get 
nsec-related errors, even when using -DGIT_USE_NSEC:

 error: ‘struct stat’ has no member named ‘st_mtime_nsec’

Perform some error checking when examining symlink directories.