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.
adding bar
adding foo
% state dump after
n 666 -1 bar
n 666 -1 foo
% status
! bar
? baz
C foo