bundle: exit early when there are no commits to bundle

Previously, if you passed a revset that resolved to no nodes, it would get
interpreted by the changegroup discovery logic as 'bundle all my heads', which
is not what the user asked.

Let's exit early when we notice this case.

It could be argued that the changeset discovery logic should be smarter and only
assume 'all heads' if the incoming heads parameter is None, but that's a much
riskier change.

mercurial/commands.py

@@ -1328,7 +1328,10 b' def bundle(ui, repo, fname, dest=None, *'
     """
     revs = None
     if 'rev' in opts:
-        revs = scmutil.revrange(repo, opts['rev'])
+        revstrings = opts['rev']
+        revs = scmutil.revrange(repo, revstrings)
+        if revstrings and not revs:
+            raise error.Abort(_('no commits to bundle'))
 
     bundletype = opts.get('type', 'bzip2').lower()
     try:

tests/test-bundle.t

@@ -728,4 +728,8 b' bundle single branch'
   checking files
   4 files, 3 changesets, 5 total revisions
 
-  $ cd ..
+== Test bundling no commits
+
+  $ hg bundle -r 'public()' no-output.hg
+  abort: no commits to bundle
+  [255]