convert: fix svn branch source detection corner case
It was confused by composite-looking revisions made by copy + remove.
#!/bin/sh
hg init a
cd a
echo a > a
hg add -n
hg st
hg add
hg st
echo b > b
hg add -n b
hg st
hg add b || echo "failed to add b"
hg st
echo % should fail
hg add b
hg st
hg ci -m 0
echo % should fail
hg add a
echo aa > a
hg ci -m 1
hg up 0
echo aaa > a
hg ci -m 2
hg merge
hg st
echo % should fail
hg add a
hg st
hg ci -m merge
echo % issue683
hg rm a
hg st
echo a > a
hg add a
hg st
hg add c && echo "unexpected addition of missing file"
echo c > c
hg add d c && echo "unexpected addition of missing file"
hg st