am: use get_author_ident_from_commit instead of mailinfo when rebasing

am: use get_author_ident_from_commit instead of mailinfo when rebasing

In certain situations, commit authorship can consist of an invalid
e-mail address. For example, this is the case when working with git svn
repos where the author email has had the svn repo UUID appended such as:

 author@example.com <author@example.com@deadbeef-dead-beef-dead-beefdeadbeef>

Given such an address, mailinfo extracts the authorship incorrectly as
it assumes a valid domain. However, when rebasing the original
authorship should be preserved irrespective of its validity as an email
address.

Using get_author_ident_from_commit instead of mailinfo when rebasing
preserves the original authorship.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>