sequencer: fix "rebase -i --root" corrupting author header timezone

sequencer: fix "rebase -i --root" corrupting author header timezone

When "git rebase -i --root" creates a new root commit, it corrupts the
"author" header's timezone by repeating the last digit:

    author A U Thor <author@example.com> @1112912773 -07000

This is due to two bugs.

First, write_author_script() neglects to add the closing quote to the
value of GIT_AUTHOR_DATE when generating "rebase-merge/author-script".

Second, although sq_dequote() correctly diagnoses the missing closing
quote, read_author_ident() ignores sq_dequote()'s return value and
blindly uses the result of the aborted dequote.

sq_dequote() performs dequoting in-place by removing quoting and
shifting content downward. When it detects misquoting (lack of closing
quote, in this case), it gives up and returns an error without inserting
a NUL-terminator at the end of the shifted content, which explains the
duplicated last digit in the timezone.

(Note that the "@" preceding the timestamp is a separate bug which
will be fixed subsequently.)

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>