patchbomb: make `hg email` reusable for other patch sources Adds two internal options, patches and patchnames, which allow other extensions to email a given set of patches. The pbranch extension needs this to send its patches which are diffs between topic branches.

patchbomb: make options to email command reusable Makes the option set that controls the actual emailing of the patches available as patchbomb.emailopts.

Backed out changeset c5dbe86b0fee (issue1375)