dim: make dry-run a bit more useful for pull requests

dim -d pull-request was failing with:

	Using drm/drm-next as the upstream
	../maintainer-tools/dim: line 2225: gitk: command not found
	git tag -F- drm-xe-next-2024-02-27 drm-xe-next@{upstream}
	git tag -a -f drm-xe-next-2024-02-27
	error: src refspec drm-xe-next-2024-02-27 does not match any
	error: failed to push some refs to 'ssh://gitlab.freedesktop.org/drm/xe/kernel.git'

Make it a little bit more useful. Probably we shouldn't skip that much,
but that can be left for later. Same command now shows:

	Using drm/drm-next as the upstream
	../maintainer-tools/dim: line 2229: gitk: command not found
	git tag -F- drm-xe-next-2024-02-27 drm-xe-next@{upstream}
	git tag -a -f drm-xe-next-2024-02-27
	To ssh://gitlab.freedesktop.org/drm/xe/kernel.git


	 * [new tag]                     b2121f2bd2232cd0556b2182078d159d81497885 -> drm-xe-next-2024-02-27
	Skip.
	neomutt -s [PULL] drm-xe-next -i /tmp/tmp.eu5PC10boN -c Jani Nikula <jani.nikula@linux.intel.com> -c Joonas Lahtinen <joonas.lahtinen@linux.intel.com> -c Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> -c Rodrigo Vivi <rodrigo.vivi@intel.com> -c Thomas Zimmermann <tzimmermann@suse.de> -c Maarten Lankhorst <maarten.lankhorst@linux.intel.com> -c Maxime Ripard <mripard@kernel.org> -c Thomas Hellström <thomas.hellstrom@linux.intel.com> -c Oded Gabbay <ogabbay@kernel.org> -c Lucas De Marchi <lucas.demarchi@intel.com> -c dri-devel@lists.freedesktop.org -c intel-gfx@lists.freedesktop.org -c intel-xe@lists.freedesktop.org -c dim-tools@lists.freedesktop.org Dave Airlie <airlied@gmail.com> Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>