webcommands: stop using ersatz if-else ternary operator for rename variable 6ddc86eedc3b didn't remove it, let's do it now. Placing the added lines into the already existing "if fctx is not None" block also makes webcommands.comparison() look a bit more like webcommands.filediff(), which eases possible future refactoring. And fctx is not None only when path in ctx, so logically it's equivalent.

webcommands: get correct parents when comparing a removed file (issue4962) When comparing a file that was removed at the current revision, parents used to show grandparents instead, due to how fctx was "shifted" from the current revision to its p1. Let's not do that. The fix is pretty much copied from webcommands.filediff().