patch: extend filtestore to store an optional copy source This will help wrapping filestores in memctx.

paper/coal: use fixed width for diffstat +/- link The plus and minus characters are normally not the same width in a non-monospace font, and this made the line length change when the diffstat display was toggled. The square brackets are not rendered in a monospace font to ensure that they align with the parenthesis on the same line.

hg: rename opts argument to peeropts in clone This should make it clearer that the options are used with remoteui only and are not used otherwise in hg.clone.