Include GIT_ASSERT_WITH_RETVAL and GIT_ASSERT_ARG_WITH_RETVAL so that
functions that do not return int (or more precisely, where `-1` would
not be an error code) can assert.

This allows functions that return, eg, NULL on an error code to do that
by passing the return value (in this example, `NULL`) as a second
parameter to the GIT_ASSERT_WITH_RETVAL functions.

Fall back to the system assert(3) in debug builds, which may aide
in debugging.

"Safe" assertions can be enabled in debug builds by setting
GIT_ASSERT_HARD=0.  Similarly, hard assertions can be enabled in
release builds by setting GIT_ASSERT_HARD to nonzero.

Provide macros to replace usages of `assert`.  A true `assert` is
punishing as a library.  Instead we should do our best to not crash.

GIT_ASSERT_ARG(x) will now assert that the given argument complies to
some format and sets an error message and returns `-1` if it does not.

GIT_ASSERT(x) is for internal usage, and available as an internal
consistency check.  It will set an error message and return `-1` in the
event of failure.

Fix uninitialized stack memory and NULL ptr dereference in stash_to_index

Caught by static analysis.

The checkout code didn't iterate into a subdir if it didn't match the
pathspec, but since the pathspec might match files in the subdir we
should recurse into it (In contrast to gitignore handling).

Fixes #5089

Honor GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH for all checkout types 

We were previously applying the pathspec filter for the baseline
iterator during checkout, as well as the target tree.  This was an
oversight; in fact, we should apply the pathspec filter to _all_
checkout targets, not just trees.

Add a helper function to set the iterator pathspecs from the given
checkout pathspecs, and call it everywhere.

The checkout::index::can_disable_pathspec_match test attempts to set a
path filter of `test11.txt` and `test12.txt`, but then validates that
`test10.txt` and `test11.txt` were left unmodified.  Update the test's
path filter to match the expectation.