am: Fix launching of pager

am: Fix launching of pager

The pagination functionality in git am has some problems:

 - It does not check if stdout is a tty, so it always paginates.

 - If $GIT_PAGER uses any environment variables, they are being
   ignored, since it does not run $GIT_PAGER through eval.

 - If $GIT_PAGER is set to the empty string, instead of passing
   output through to stdout, it tries to run $dotest/patch.

Fix them.  While at it, move the definition of git_pager() to
git-sh-setup so authors of other commands are not tempted to
reimplement it with the same mistakes.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>