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.

(cherry picked from commit ea19efc1)