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

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

When "git rebase -i --root" creates a new root commit (say, by swapping
in a different commit for the root), it corrupts the commit's "author"
header with trailing garbage:

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

This is a result of read_author_ident() neglecting to NUL-terminate the
buffer into which it composes the "author" header.

(Note that the "@" preceding the timestamp and the extra "0" in the
timezone are separate bugs which will be fixed subsequently.)

Security considerations: Construction of the "author" header by
read_author_ident() happens in-place and in parallel with parsing the
content of "rebase-merge/author-script" which occupies the same buffer.
This is possible because the constructed "author" header is always
smaller than the content of "rebase-merge/author-script". Despite
neglecting to NUL-terminate the constructed "author" header, memory is
never accessed (either by read_author_ident() or its caller) beyond the
allocated buffer since a NUL-terminator is present at the end of the
loaded "rebase-merge/author-script" content, and additional NUL's are
inserted as part of the parsing process.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>