Introduce `git_filter_list__convert_buf` which behaves like the old
implementation of `git_filter_list__apply_data`, where it might move the
input data buffer over into the output data buffer space for efficiency.

This new implementation will do so in a more predictible way, always
freeing the given input buffer (either moving it to the output buffer or
filtering it into the output buffer first).

Convert internal users to it.

The new git_repository_workdir_path function does error checking on
working directory inputs on Windows; use it to construct paths within
working directories.

This change:

* Initializes a few variables that were being read before being
  initialized.
* Includes https://github.com/madler/zlib/pull/393. As such,
  it only works reliably with `-DUSE_BUNDLED_ZLIB=ON`.

libgit2 does not use `type_t` suffixes as it's redundant; thus, rename
`git_iterator_type_t` to `git_iterator_t` for consistency.

Instead of using a signed type (`off_t`) use `uint64_t` for the maximum
size of files.

Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

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

The only function that is named `issomething` (without underscore) was
`git_oid_iszero`.  Rename it to `git_oid_is_zero` for consistency with
the rest of the library.

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