merge: handle case when heads are all bookmarks
If all heads are bookmarks, merge fails to find what node to merge
with (throws an IndexError while indexing into the non-bookmark heads
list) as of
4a02cf4fbb2e. This catches that case and prints an error
to specify a rev explicitly.
verify: do not choke on valid changelog without manifest
Before this change:
$ hg init
$ hg branch foo
$ hg ci -m branchfoo
$ hg verify
checking changesets
checking manifests
0: empty or missing manifest
crosschecking files in changesets and manifests
checking files
0 files, 1 changesets, 0 total revisions
1 integrity errors encountered!
(first damaged changeset appears to be 0)
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