get_maintainers.pl: improve .mailmap parsing

Entries that used formats other than "Proper Name <commit@email.xx>"
were not parsed properly.

Try to improve the parsing so that the entries in the forms of:
    Proper Name <proper@email.xx> <commit@email.xx>
and
    Proper Name <proper@email.xx> Commit Name <commit@email.xx>
are transformed correctly.

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Florian Mickler <florian@mickler.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>