Include the SHA1 collision attack detection library from
https://github.com/cr-marcstevens/sha1collisiondetection

cmake: only enable supported compiler warning flags

We currently unconditionally enable the "-Wall" and "-Wextra" flags.
Some platforms rely on compilers which do not support these flags,
though. One of these platforms is Haiku, which does not support
"-Wextra" due to being stuck on GCC version 2.

Fix builds on such platforms by adding these flags only if supported by
the compiler.

tests: refs::create: fix memory leak

Fix: make reflog include "(merge)" for merge commits

This test ensures that the string '(merge)' is included in the reflog
when a merge commit is made.

This fixes issue #4094

tests: Add create__symbolic_with_arbitrary_content

Fix minor typos in CONVENTIONS.md

Fix inet_pton tests triggering an assert in Haiku

Haiku will assert in a nightly build if the "dst" input to inet_pton() is NULL.

Use C style comments as per style guide,
and fix mismatching indentation.

This test ensures that it's possible to create a symbolic ref that
has arbitrary data as its target. It also ensures it's possible
to obtain the target of that symbolic reference from the git_reference
object.

appveyor: don't rewrite the system mingw

Download mingw-w64 into our build directory and execute it there, don't
try to overwrite the system's mingw.

The 'appveyor' branch is useful for testing AppVeyor builds.

fix regression from #4092

Fixes a regression from #4092. This is a crash on 32-bit and I assume that
it doesn't do the right thing on 64-bit either. MSVC emits a warning for this,
but of course, it's easy to get lost among all of the similar 'possible loss
of data' warnings.

Attrcache cleanups

Provide error on gcc < 4.1 when THREADSAFE

Provide a descriptive error message when compiling THREADSAFE on gcc
versions < 4.1.  We require the atomic primitives (eg
`__sync_synchronize`) that were introduced in that version.

(Note, clang setes `__GNUC__` but appears to set its version > 4.1.)

Remove useless indirection from `git_attr_cache__init` to
`git_attr_cache__do_init`. The difference is that the
`git_attr_cache__init` macro first checks if the cache is already
initialized and, if so, not call `git_attr_cache__do_init`. But
actually, `git_attr_cache__do_init` already does the same thing and
returns immediately if the cache is already initialized.

Remove the indirection.

When doing an upsert of a file, we used to use `git__compare_and_swap`,
comparing the entry's file which is to be replaced with itself. This can
be more easily formulated by using `git__swap`, which unconditionally
replaces the value.

Improve encapsulation by not referencing the attrcache mutex directly
but instead using the `attr_cache_lock` and `attr_cache_unlock`
functions.

Improve clar messages

Provide more detailed messages when conditions pass or fail
unexpectedly.  In particular, this provides the error messages when a
test fails with a different error code than was expected.

`snprintf` requires a _format_ but does not require _arguments_ to the
format.  eg: `snprintf(buf, 42, "hi")` is perfectly legal.  Expand the
macro to match.

Without this, `p_sprintf(buf, 42, "hi")` errors with:

```
error: expected expression
                p_snprintf(msg, 42, "hi");
                ^
src/unix/posix.h:53:34: note: expanded from macro 'p_snprintf'
                                 ^
/usr/include/secure/_stdio.h:57:73: note: expanded from macro 'snprintf'
  __builtin___snprintf_chk (str, len, 0, __darwin_obsz(str),
__VA_ARGS__)
```

khash cleanups