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).

trydiff: read file data in only one place This moves getfilectx() out of the initial block in the loop, leaving that block to be only about finding pairs of filenames in ctx1 and ctx2 to diff.

trydiff: set filename variables to None even when not opts.git or losedatafn There is not much left of the first block "if opts.git or losedatafn" block now. The next patch will move the call to getfilectx() out of that block. We will then be using the defined-ness of 'f1' to tell whether the file existed in ctx1 (and under what name). We will need this information whether or not opts.git or losedatafn was set, so just remove that guard. The only operation in the block that is not cheap is the call to getfilectx(), but that has an extra 'if opts.git' guard already. --ignore-space-change proves that only 'if opts.git or losedatafn:' was removed.

trydiff: read flags in one place Now that we can trust f1/f2 to indicate whether that side of the diff exists, we can move the calls to ctx.flag() to a single place.