echo Hello > a.txt
fossil add a.txt
fossil commit -m "new file"
echo newline >> a.txt
fossil commit --branch b -m "new branch with a new line"
fossil update trunk
fossil mv a.txt b.txt
mv a.txt b.txt
fossil commit "rename on trunk"
fossil merge b
# It does not warn about anything, considers the merge clean
# and the file b.txt does not have the new line on a.txt

anonymous added on 2010-12-09 21:35:09:
Merging in the other direction, from "b" running "merge trunk", also ends with the file renamed but the new line lost.

This seems to have been fixed in the mean time. In my tests the result is:

[stephan@host:~/tmp/f/x]$ cat b.txt 
Hello
newline