fast-import: check committer name more strictly

fast-import: check committer name more strictly

The documentation declares following identity format:
(<name> SP)? LT <email> GT
where name is any string without LF and LT characters.
But fast-import just accepts any string up to first GT
instead of checking the whole format, and moreover just
writes it as is to the commit object.

git-fsck checks for [^<\n]* <[^<>\n]*> format. Note that the
space is mandatory. And the space quirk is already handled via
extending the string to the left when needed.

Modify fast-import input identity format to a slightly stricter
one - deny LF, LT and GT in both <name> and <email>. And check
for it.

This is stricter then git-fsck as fsck accepts "Name> <email>"
currently, but soon fsck check will be adjusted likewise.

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