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

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

When "git rebase -i --root" creates a new root commit, it corrupts the
"author" header's timestamp by prepending a "@":

    author A U Thor <author@example.com> @1112912773 -0700

The commit parser is very strict about the format of the "author"
header, and does not allow a "@" in that position.

The "@" comes from GIT_AUTHOR_DATE in "rebase-merge/author-script",
signifying a Unix epoch-based timestamp, however, read_author_ident()
incorrectly allows it to slip into the commit's "author" header, thus
corrupting it.

One possible fix would be simply to filter out the "@" when constructing
the "author" header timestamp, however, a more correct fix is to parse
the GIT_AUTHOR_DATE date (via parse_date()) and format the parsed result
into the "author" header. Since "rebase-merge/author-script" may be
edited by the user, this approach has the extra benefit of catching
other potential timestamp corruption due to hand-editing.

We can do better than calling parse_date() ourselves and constructing
the "author" header manually, however, by instead taking advantage of
fmt_ident() which does this work for us.

The benefits of using fmt_ident() are twofold. First, it simplifies the
logic considerably by allowing us to avoid the complexity of building
the "author" header in parallel with and in the same buffer from which
"rebase-merge/author-script" is being parsed. Instead, fmt_ident() is
invoked to compose the header after parsing is complete.

Second, fmt_ident() is careful to prevent "crud" from polluting the
composed ident. As with validating GIT_AUTHOR_DATE, this "crud"
avoidance prevents other (possibly hand-edited) bogus author information
from "rebase-merge/author-script" from corrupting the commit object.

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