Split authorship lines from the right instead of from the left

Git authorship lines are in the form 'author Name <em@i.l> timestamp timezone'.
Some clients mess up the 'Name <em@i.l>' part badly, for instance by setting two
email addresses. Splitting identity and timestamp by looking at the '> ' part
from the right instead of the left helps parse some of those messed up commits.

Such commits still fail the check() method (and they raise a warning in git fsck
upstream as well), but we can at least work with them.

(This edge case brought to you by https://forge.softwareheritage.org/T1280)