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
Differential Revision: https://phab.mercurial-scm.org/D3003