histedit: have dropmissing abort on empty plan

We noticed that many users have the intuition of laving the editor empty when
they want to abort the operation. The fact that dropmissing allows user to
delete all edited commits is not intuitive even for users that asked for it.

Let's prevent people from this footgun.

hgext/histedit.py

@@ -1379,6 +1379,10 b' def verifyactions(actions, state, ctxs):'
     missing = sorted(expected - seen)  # sort to stabilize output
 
     if state.repo.ui.configbool('histedit', 'dropmissing'):
+        if len(actions) == 0:
+            raise error.ParseError(_('no rules provided'),
+                    hint=_('use strip extension to remove commits'))
+
         drops = [drop(state, node.bin(n)) for n in missing]
         # put the in the beginning so they execute immediately and
         # don't show in the edit-plan in the future

tests/test-histedit-drop.t

@@ -153,6 +153,10 b' Drop the last changeset'
   hg: parse error: missing rules for changeset a4f7421b80f7
   (use "drop a4f7421b80f7" to discard, see also: "hg help -e histedit.config")
   $ hg --config histedit.dropmissing=True histedit  cb9a9f314b8b --commands - 2>&1 << EOF | fixbundle
+  > EOF
+  hg: parse error: no rules provided
+  (use strip extension to remove commits)
+  $ hg --config histedit.dropmissing=True histedit  cb9a9f314b8b --commands - 2>&1 << EOF | fixbundle
   > pick cb9a9f314b8b a
   > pick ee283cb5f2d5 e
   > EOF