git-pull.sh: better warning message for "git pull" on detached head.
git-pull.sh: better warning message for "git pull" on detached head.

Otherwise, git complains about not finding a branch to pull from in
'branch..merge', which is hardly understandable. While we're there,
reword the sentences slightly.

Signed-off-by: Matthieu Moy <d4d2cc2a6ec7b2aa587d4887cc5ed200f9b685a1@imag.fr>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git