This change uses compiler intrinsics to detect overflows instead of
using divisions to detect potential overflow. This makes the code faster
and makes it easier to read as a bonus side-effect!
Some of the things this quickens:
* Config parsing.
* Tree parsing.
* Smart protocol negotiation.
\* Measured by running `libgit2_clar` with `-fno-optimize-sibling-calls
-fno-omit-frame-pointer` under `perf(1)`:
```shell
$ perf diff --symbols=git__strntol64 --compute=ratio \
--percentage=absolute baseline.data perf.data
\# Event 'cycles'
\#
\# Baseline Ratio Shared Object
\# ........ .............. .............
\#
0.25% 0.321836 libgit2_clar
```
Provide more clarity for Win32 calling conventions - now that we always compile to __cdecl on Win32, we'll define that as the the libgit2 calling convention. Also offer NTAPI (__stdcall) calling conventions for things that need callbacks from NTAPI code (eg fiber-local storage).
The number of CPUs is useful information for creating a thread pool or a number of workers, but it's not really about threading directly. Evict it from the thread file
We've accumulated quite some functions which are never used outside of their respective code unit, but which are lacking the `static` keyword. Add it to reduce their linkage scope and allow the compiler to optimize better.
Our hand-rolled fallback sorting function `git__insertsort_r` does an in-place sort of the given array. As elements may not necessarily be pointers, it needs a way of swapping two values of arbitrary size, which is currently implemented by allocating a temporary buffer of the element's size. This is problematic, though, as the emulated `qsort` interface doesn't provide any return values and thus cannot signal an error if allocation of that temporary buffer has failed. Convert the function to swap via a temporary buffer allocated on the stack. Like this, it can `memcpy` contents of both elements in small batches without requiring a heap allocation. The buffer size has been chosen such that in most cases, a single iteration of copying will suffice. Most importantly, it can fully contain `git_oid` structures and pointers. Add a bunch of tests for the `git__qsort_r` interface to verify nothing breaks. Furthermore, this removes the declaration of `git__insertsort_r` and makes it static as it is not used anywhere else.
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.