tests/test-debugindexdot.t

convert: don't drop missing or corrupt tag entries

Cleaning up the tags file could be a useful feature in some cases, so maybe
there should be a switch for this.  However, the default hg -> hg convert tries
to maintain identical hashes (thus convert.hg.saverev is off by default, but is
on by default for other source types).  It looks like _rewritesubstate() has a
`continue` in it, and therefore a similar problem.

I ran into this conversion divergence when a coworker "merged" two repositories
by copy/pasting all of the files from the source repo and massaging the code,
and forgetting to revert the .hg* files.  That silently emptied the .hgtags file
after the conversion.  (This isn't the manifest node bug Yuya has been helping
with- this occurred well after the bzr -> hg conversion and wasn't a merge
commit, which made it extra puzzling.  That bug is still an issue.)

Just exercise debugindexdot
Create a short file history including a merge.
  $ hg init t
  $ cd t
  $ echo a > a
  $ hg ci -qAm t1 -d '0 0'
  $ echo a >> a
  $ hg ci -m t2 -d '1 0'
  $ hg up -qC 0
  $ echo b >> a
  $ hg ci -m t3 -d '2 0'
  created new head
  $ HGMERGE=true hg merge -q
  $ hg ci -m merge -d '3 0'

  $ hg debugindexdot a
  digraph G {
  	-1 -> 0
  	0 -> 1
  	0 -> 2
  	2 -> 3
  	1 -> 3
  }

  $ cd ..