Fix and improve t7004 (git-tag tests)

Fix and improve t7004 (git-tag tests)

Brown paper bag fix to avoid using non portable sed syntax. The
test by itself didn't catch what it was supposed to, anyways.

The new test first checks if git-tag correctly errors out when
the user exited the editor without editing the file.  Then it
checks if what the user was presented in the editor was any
useful, which we define as the following:

 * It begins with a single blank line, where the invoked editor
   would typically place the editing curser at, so that the user
   can immediately start typing;

 * It has some instruction but that comes after that initial
   blank line, all lines prefixed with "#".  We specifically do
   not check for the wording of this instruction.

 * And it has nothing else, as the expected behaviour is "Hey
   you did not leave any message".

Signed-off-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>