dim: Check that patches have git-format-patch default prefixes

By default the git-format-patch command generates patches with prefixes
for the source and destination (-p1), and is also what git-am uses as a
default. The command strips the first leading path component when patch
is applied (unless a different -p<num> argument is used).

But the patch generating behaviour can be changed with git-format-patch
--no-prefix argument (or setting 'diff.noprefix = true' in .gitconfig).

Patches with no source and destination prefixes can confuse the git-am
3-way merge logic, since stripping the first path component will lead
to wrong paths for newly added files.

To avoid this, check that patches to apply are using git-format-patch's
defaults prefixes to make sure that git-am defaults are safe to use too.

Since there are developers that generate patches without using git, make
this an error but allow to still continue when using the dim -f option.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>