stringutil: improve check for failed mailmap line parsing The existing check for a bad mailmap file entry fails with inputs like b'>@<'. This commit adds a function to check if a sufficient amount of information has been parsed from a mailmap file entry. At minimum, one email must be found (assumed to be the commit email). If email is not empty and no names are found, then there must be two emails. If there are at least one email and name, the mapping is valid. Differential Revision: https://phab.mercurial-scm.org/D3003