When an integer that is parsed with `git__strntol32` is too big to fit
into an int32, we will generate an error message that includes the
actual string that failed to parse. This does not acknowledge the fact
that the string may either not be NUL terminated or alternative include
additional characters after the number that is to be parsed. We may thus
end up printing characters into the buffer that aren't the number or,
worse, read out of bounds.

Fix the issue by utilizing the `endptr` that was set by
`git__strntol64`. This pointer is guaranteed to be set to the first
character following the number, and we can thus use it to compute the
width of the number that shall be printed. Create a test to verify that
we correctly truncate the number.

Some edge cases were currently completely untested, e.g. parsing numbers
greater than INT64_{MIN,MAX}, truncating buffers by length and invalid
characters. Add tests to verify that the system under test performs as
expected.

The function `git__strtol32` can easily be misused when untrusted data
is passed to it that may not have been sanitized with trailing `NUL`
bytes. As all usages of this function have now been removed, we can
remove this function altogether to avoid future misuse of it.

The function `git__strtol64` does not take a maximum buffer length as
parameter. This has led to some unsafe usages of this function, and as
such we may consider it as being unsafe to use. As we have now
eradicated all usages of this function, let's remove it completely to
avoid future misuse.

Clay is the name of a programming language on the makings, and we want
to avoid confusions. Sorry for the huge diff!

The clay script didn't match the latest version from upstream.
Additionaly, add core/strtol.c to complete porting the core tests to
clay.

Signed-off-by: schu <schu-github@schulog.org>