fast-import: don't fail on omitted committer name

fast-import: don't fail on omitted committer name

fast-import format declares 'committer_name SP' to be optional in
'committer_name SP LT email GT'. But for a (commit) object SP is
obligatory while zero length committer_name is ok. git-fsck checks
that SP is present, so fast-import must prepend it if the name SP
part is omitted. It doesn't do so and thus for "LT email GT" ident
it writes a bad object.

Name cannot contain LT or GT, ident always comes after SP in fast-import.
So if ident starts with LT reuse the SP as if a valid 'SP LT email GT'
ident was passed.

This fixes a ident parsing bug for a well-formed fast-import input.
Though the parsing is still loose and can accept a ill-formed input.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>