We use `git__strtol64` and `git__strtol32` to parse the trailing commit
or author date and timezone of signatures. As signatures are usually
part of a commit or tag object and thus essentially untrusted data, the
buffer may be misformatted and may not be `NUL` terminated. This may
lead to an out-of-bounds read.

Fix the issue by using `git__strntol64` and `git__strntol32` instead.