merge: fix spurious merges for copies in linear updates
We make better use of contexts to accurately identify copies that
don't need merges.
Add a simple test and update other tests.
200 Script output follows
content-type: text/plain
content-length: 157
content-disposition: filename=sometext.txt
This is just some random text
that will go inside the file and take a few lines.
It is very boring to read, but computers don't
care about things like that.
host - - [date] "GET /?f=f165dc289438;file=sometext.txt;style=raw HTTP/1.1" 200 -