Don't invoke msgcat needlessly when updating

When updating from source code, don't call msgcat just to "merge" single
file - effectively just copy it, more expensively, to a new file.
Optimize this common(!) case and avoid calling msgcat; only run it to
concatenate 2+ files.
Update Sparkle

Include fix to avoid excessive logging.
Better error when updating w/o source code

A common reason for failure of Update from sources is that there's no
source code - the translator clicks the button without realizing what
it's for. Instead of returning a series of increasingly scary errors,
detect this case and provide a hopefully better understandable
explanation that the source code is missing.