dagop: fix subsetparentswalker to set p1/p2 chains at merge revision
The previous implementation was wrong because the '1'/'2' key would be
appended at a fork revision. Since we traverse the graph from heads, a merge
revision is actually a branching point, where the sort key must be generated.
#require symlink
https://bz.mercurial-scm.org/1438
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch