[PATCH] Rename text-base variables in update-editor.c

I'm going to apply this patch or something like it, tomorrow. The new
names are very helpful in matching up text-base checksums to the
corresponding (old, current, new, copied, revert, ...) text base paths.

Trawling through all this is also helping me to understand the text-base
manipulations in the update editor, and my next move here will be to
replace the old text-base paths with new WC-NG pristine store functions,
which is now looking pretty straightforward in this part of the code.
I'll just do any stream wrapping necessary to ensure the MD5 checksums
are still calculated as well as the new SHA1s.

Any comments about this renaming? A mass rename could affect anybody's
patches in progress but it should be simple enough to merge. Let me
know if there's a particular reason to hold off for a short time.

- Julian