Split authorship lines from the right instead of from the left

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)