fast-import: add input format tests

fast-import: add input format tests

Documentation/git-fast-import.txt says that git-fast-import is strict
about it's input format. But committer/author field parsing is a bit
loose. Invalid values can be unnoticed and written out to the commit,
either with format-conforming input or with non-format-conforming one.

Add one passing and one failing test for empty/absent committer name
with well-formed input. And a failed test with unnoticed ill-formed
input.

Reported-by: SASAKI Suguru <sss.sonik@gmail.com>
Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>