shellquote: fix missing quotes for empty string "hg kdiff3 -rREV" did not work because 1642eb429536 and 5edb387158a1 failed to handle empty argument.

trydiff: transpose 'if opts.git or losedatafn' with 'if f[12]' Perhaps it's more readable this way...

trydiff: extract function that generates filename pairs The code that identifies copies/renames, as well as the filenames before and after, is now isolated and we can extract it to a function so it can be overridden by extensions (in particular the narrow clone extension).