git-svn: trim leading and trailing whitespaces in author name

git-svn: trim leading and trailing whitespaces in author name

In some cases, the svn author names might contain leading or trailing
whitespaces, leading to messages such as:

  Author: user1
   not defined in authors.txt

(the trailing newline leads to the line break). The user "user1" is
defined in authors.txt though, e.g.

  user1 = User <redacted>

Fix this by trimming the author name retreived from svn before using it
in check_author.

Helped-by: Eric Sunshine <redacted>
Signed-off-by: Tobias Klauser <redacted>
Signed-off-by: Junio C Hamano <redacted>