revert: fix corner case found by faheem mitha.
if file not in dest manifest, remove only if in source manifest.
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
removing b
this update spans a branch affecting the following files:
b
aborting update spanning branches!
(use 'hg merge' to merge across branches or 'hg update -C' to lose changes)