makefile: rename debian-jessie to deb

`make debian-jessie` was a poor name choice on my part, because it
really builds a package for the current debian platform, whatever that
happens to be, rather than a specific platform. Rename it to just
`make deb` so that it's a little clearer. As a result, it now places
built debs in a "debian-unknown" directory.

Makefile

@@ -157,10 +157,10 b' osx:'
 	N=`cd dist && echo mercurial-*.mpkg | sed 's,\.mpkg$$,,'` && hdiutil create -srcfolder dist/$$N.mpkg/ -scrub -volname "$$N" -ov packages/osx/$$N.dmg
 	rm -rf dist/mercurial-*.mpkg
 
-debian-jessie:
-	mkdir -p packages/debian-jessie
+deb:
+	mkdir -p packages/debian-unknown
 	contrib/builddeb
-	mv debbuild/*.deb packages/debian-jessie
+	mv debbuild/*.deb packages/debian-unknown
 	rm -rf debbuild
 
 docker-debian-jessie: