packaging: use /usr/bin/python3 shebang for scripts in Debian

"hg" script is already correct because it is handled by setup.py but
"hg-ssh" will be rewritten by dh_python into "/usr/bin/python" which is
not wanted as we target Python 3. By passing --shebang=/usr/bin/python3
to dh_python3, we force shebangs to be set with this value.

contrib/packaging/debian/rules

@@ -1,47 +1,47
 #!/usr/bin/make -f
 # Uncomment this to turn on verbose mode.
 # export DH_VERBOSE=1
 
 CPUS=$(shell cat /proc/cpuinfo | grep -E ^processor | wc -l)
 
 export HGPYTHON3=1
 export PYTHON=python3
 
 %:
 	dh $@ --with python3
 
 override_dh_auto_test:
 	http_proxy='' dh_auto_test -- TESTFLAGS="-j$(CPUS)"
 
 override_dh_python3:
-	dh_python3
+	dh_python3 --shebang=/usr/bin/python3
 	find debian/mercurial/usr/share -type d -empty -delete
 
 override_dh_install:
 	python3 setup.py install --root "$(CURDIR)"/debian/mercurial --install-layout=deb
 	# chg
 	make -C contrib/chg \
 		DESTDIR="$(CURDIR)"/debian/mercurial \
 		PREFIX=/usr \
 		clean install
 	# remove arch-independent python stuff
 	find "$(CURDIR)"/debian/mercurial/usr/lib \
 		! -name '*.so' ! -type d -delete , \
 		-type d -empty -delete
 	python3 setup.py install --root "$(CURDIR)/debian/mercurial-common" --install-layout=deb
 	make install-doc PREFIX="$(CURDIR)"/debian/mercurial-common/usr
 	# remove arch-dependent python stuff
 	find "$(CURDIR)"/debian/mercurial-common/usr/lib \
 		-name '*.so' ! -type d -delete , \
 		-type d -empty -delete
 	cp contrib/hg-ssh "$(CURDIR)"/debian/mercurial-common/usr/bin
 	mkdir -p "$(CURDIR)"/debian/mercurial-common/usr/share/mercurial
 	cp contrib/hgk "$(CURDIR)"/debian/mercurial-common/usr/share/mercurial
 	mkdir -p "$(CURDIR)"/debian/mercurial-common/etc/mercurial/hgrc.d/
 	cp contrib/packaging/debian/*.rc "$(CURDIR)"/debian/mercurial-common/etc/mercurial/hgrc.d/
 	# completions
 	mkdir -p "$(CURDIR)"/debian/mercurial-common/usr/share/bash-completion/completions
 	cp contrib/bash_completion "$(CURDIR)"/debian/mercurial-common/usr/share/bash-completion/completions/hg
 	mkdir -p "$(CURDIR)"/debian/mercurial-common/usr/share/zsh/vendor-completions
 	cp contrib/zsh_completion "$(CURDIR)"/debian/mercurial-common/usr/share/zsh/vendor-completions/_hg
 	rm "$(CURDIR)"/debian/mercurial-common/usr/bin/hg