deb: install zsh completions to /usr/share/zsh/vendor-completions

This location is used by debian (and ubuntu) to store completions provided by
other deb packages.  The default fpath appears to have this before any of the
zsh-provided instances of the completions, so this should take precedence.

Differential Revision: https://phab.mercurial-scm.org/D882

contrib/debian/rules

@@ -33,4 +33,6 b' override_dh_install:'
 	cp contrib/debian/*.rc $(CURDIR)/debian/mercurial-common/etc/mercurial/hgrc.d/
 	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

tests/test-debian-packages.t

@@ -21,3 +21,6 b' main deb should have .so but no .py'
 mercurial-common should have py but no .so or pyc
   $ dpkg --contents mercurial-common_*.deb | egrep '(localrepo|parsers.*so)'
   * ./usr/lib/python2.7/dist-packages/mercurial/localrepo.py (glob)
+zsh completions should be in the common package
+  $ dpkg --contents mercurial-common_*.deb | egrep 'zsh.*[^/]$'
+  * ./usr/share/zsh/vendor-completions/_hg (glob)