We keep the treebuilder entrycount as a `size_t` - return that instead
of downcasting to an `unsigned int`.  Callers who were storing this
value in an `unsigned int` will continue to downcast themselves, so
there should be no behavior change for callers.

The `git__utf8_charlen` now takes `size_t` as the buffer length, since
it contains the full length of the buffer at the current position.  It
now returns `-1` in all cases where utf8 codepoints are invalid, since
callers only care about a valid length of a sequence of codepoints, or
if the current position is not valid utf8.

Only read at most INT_MAX from the underlying stream, so that we can
accurately return the number of bytes read.

Since callers are not guaranteed to get as many bytes as requested (due
to availability of input), this is safe and callers should call in a
loop until EOF.

Explicitly truncate the file size to a `uint32_t`.

Don't calculate 4 GiB as that will produce a compiler warning on MSVC.
Just hardcode it.

errors: use lowercase

largefile tests: only write 2GB on 32-bit platforms

Use lowercase for our error messages, per our custom.

Don't try to feed 4 GB of data to APIs that only take a `size_t` on
32-bit platforms.

Fix broken link in README

net: remove unused `git_headlist_cb`

cmake: default NTLM client to off if no HTTPS support

If building libgit2 with `-DUSE_HTTPS=NO`, then CMake will
generate an error complaining that there's no usable HTTPS
backend for NTLM. In fact, it doesn't make sense to support NTLM
when we don't support HTTPS. So let's should just have
NTLM default to OFF when HTTPS is disabled to make life easier
and to fix our OSSFuzz builds failing.

attr: rename constants and macros for consistency

Our enumeration values are not generally suffixed with `T`.  Further,
our enumeration names are generally more descriptive.

Change API instances of `fromnoun` to `from_noun` (with an underscore)

object: rename git_object__size to git_object_size

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the tag function for
consistency with the rest of the library.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the index functions for
consistency with the rest of the library.

We don't use double-underscores in the public API.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.