The `map_free` functions were not implemented as functions but instead
as macros which also set the map to NULL. While this is most certainly
sensible in most cases, we should prefer the more obvious behavior,
namingly leaving the map pointer intact.

Furthermore, this macro has been refactored incorrectly during the
map-refactorings: the two statements are not actually grouped together
by a `do { ... } while (0)` block, as it is required for macros to
match the behavior of functions more closely. This has led to at least
one subtle nesting error in `pack-objects.c`. The following code block

```
    if (pb->object_ix)
        git_oidmap_free(pb->object_ix);
```

would be expanded to

```
    if (pb->object_ix)
        git_oidmap__free(pb->object_ix); pb->object_ix = NULL;
```

which is not what one woudl expect. While it is not a bug here as it
would simply become a no-op, the wrong implementation could lead to bugs
in other occasions.

Fix this by simply removing the macro altogether and replacing it with
real function calls. This leaves the burden of setting the pointer to
NULL afterwards to the caller, but this is actually expected and behaves
like other `free` functions.

We currently call `git_strmap_free` on `checkout_data.mkdir_map` in the
`checkout_data_clear` function. The only thing protecting us from a
double-free is that the `git_strmap_free` function is in fact not a
function, but a macro that also sets the map to NULL.

Remove the second call to `git_strmap_free` and explicitly set the map
member to NULL.

submodule: catch when submodule is not staged on update

winhttp: disambiguate error messages when sending requests

When calling `git_submodule_update` on a submodule, we have to retrieve
the ID of the submodule entry in the index. If the function is called on
a submodule which is only partly initialized, the submodule entry may
not be added to the index yet. This leads to an assert when trying to
look up the blob later on.

Fix the issue by checking if the index actually holds the submodule's
ID and erroring out if it does not.

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.