tests/test-revlog-packentry.out

prepush: add more precise error messages

Part of the patch is from [e-mail]

- indicate the branch name where there are multiple heads
- give better advice when hitting a possible race, where new heads are added
  between discovery and the call to branchmap(). In that case, asking the user
  to merge isn't helpful, since only remote has the changes.

adding foo
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
adding foo
created new head
   rev    offset  length   base linkrev nodeid       p1           p2
     0         0       0      0       0 b80de5d13875 000000000000 000000000000
     1         0      24      0       1 0376abec49b8 000000000000 000000000000