histedit: crashing with a more useful error message on empty defaultrev

Before this, `hg --config histedit.defaultrev= histedit` would crash with
File "destutil.py", line 385, in desthistedit
    if revs:
UnboundLocalError: local variable 'revs' referenced before assignment

Differential Revision: https://phab.mercurial-scm.org/D5543

mercurial/destutil.py

@@ -381,6 +381,8 b' def desthistedit(ui, repo):'
         revs = stack.getstack(repo)
     elif default:
         revs = scmutil.revrange(repo, [default])
+    else:
+        raise error.Abort(_("config option histedit.defaultrev can't be empty"))
 
     if revs:
         # Take the first revision of the revset as the root

tests/test-histedit-arguments.t

@@ -112,6 +112,13 b' Test config specified default'
   > pick 08d98a8350f3 4 five
   > EOF
 
+Test invalid config default
+---------------------------
+
+  $ hg histedit --config "histedit.defaultrev="
+  abort: config option histedit.defaultrev can't be empty
+  [255]
+
 Run on a revision not descendants of the initial parent
 --------------------------------------------------------------------