winhttp: disambiguate error messages when sending requests

Skip uninteresting commits in revwalk timesort iterator

git_futils_readbuffer: don't compute sha-1

rebase: ignore untracked files in submodules

Fixes #4099

Don't compute the sha-1 in `git_futils_readbuffer_updated` unless the
checksum was requested.  This means that `git_futils_readbuffer` will
not calculate the checksum unnecessarily.

Support namespaced references again

These simple tests only ensure that we enforce the existence of a
namespace; these mirror the rugged tests, they are not exhaustive.

An untracked file in a submodule should not prevent a rebase from
starting.  Even if the submodule's SHA is changed, and that file would
conflict with a new tracked file, it's still OK to start the rebase
and discover the conflict later.

Signed-off-by: David Turner <dturner@twosigma.com>

git_commit_create: freshen tree objects in commit

Freshen the tree object that a commit points to during commit time.

Introduce (optional) SHA1 collision attack detection

We never set `SHA1_TYPE` to `builtin`.  Don't bother testing for it.

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.)