Add missing `git_reference_symbolic_create_with_log`.

Fix spelling mistake

Closes #2029

submodule branch option + little VS2013 fix

Use our strnlen on MacOS for backward compat

Apparently MacOS didn't have strnlen on 10.6 and earlier.  To
avoid having linking problems on older versions, we'll just use
our internal version.

It's exported in the headers, but the implementation was missing.

Accept 'submodule.*.fetchRecurseSubmodules' config 'on-demand' value

Updated fetch.c test to pass.

I am not sure why there was 6 in the first place.

docs: Remove non existing wrapper from the readme

libgit2net stopped to exist, all hail libgit2sharp

Reference operations with log

Overwrite ignored files on checkout

Clean up some error handling and change callback error behavior

The size_t is 32-bit already, so it overflows before going into the
function. The `-1` test should handle this gracefully in both cases
anyway.

Ok, scrap the previous commit. This is the right overflow check that
takes care of 64 bit overflow **and** 32-bit overflow, which needs to be
considered because the pool malloc can only allocate 32-bit elements in
one go.

Note that `git_pool_strdup` cannot really return any error codes,
 because the pool doesn't set errors on OOM.

 The only place where `giterr_set_oom` is called is in
 `git_pool_strndup`, in a conditional check that is always optimized
 away. `n + 1` cannot be zero if `n` is unsigned because the compiler
 doesn't take wraparound into account.

 This check has been removed altogether because `size_t` is not
 particularly going to overflow.

Validate struct versions in merge, revert

This wasn't being tested and since it has a callback, I fixed it
even though the return value of this callback is not treated like
any of the other callbacks in the API.

This renames git_vector_free_all to the better git_vector_free_deep
and also contains a couple of memory leak fixes based on valgrind
checks.  The fixes are specifically: failure to free global dir
path variables when not compiled with threading on and failure to
free filters from the filter registry that had not be initialized
fully.

This covers diff print, push, and ref foreach.  This also has a
fix for a small memory leak in the push tests.

This adds tests that try canceling an indexer operation from
within the progress callback.

After writing the tests, I wanted to run this under valgrind and
had a number of errors in that situation because mmap wasn't
working.  I added a CMake option to force emulation of mmap and
consolidated the Amiga-specific code into that new place (so we
don't actually need separate Amiga code now, just have to turn on
-DNO_MMAP).

Additionally, I made the indexer code propagate error codes more
reliably than it used to.

This time actually checking return values in diff notify tests and
actually testing callbacks for the index all-all/update-all/etc
functions.

Clone callbacks can return non-zero values to cancel the clone.
This adds some tests to verify that this actually works and updates
the documentation to be clearer that this can happen and that the
return value will be propagated back by the clone function.

The checkout notify callback behavior on non-zero return values
was not being tested.  This adds tests, fixes a bug with positive
values, and clarifies the documentation to make it clear that the
checkout can be canceled via this mechanism.