fix: warn when a fixer doesn't have a configured command

It seems we currently produce an empty command line and then decide to
not run it, but it seems better to skip that part too.

Differential Revision: https://phab.mercurial-scm.org/D7085

hgext/fix.py

@@ -800,7 +800,11 b' def getfixers(ui):'
         # dangerous to let it affect all files. It would be pointless to let it
         # affect no files. There is no reasonable subset of files to use as the
         # default.
-        if pattern is None:
+        if command is None:
+            ui.warn(
+                _(b'fixer tool has no command configuration: %s\n') % (name,)
+            )
+        elif pattern is None:
             ui.warn(
                 _(b'fixer tool has no pattern configuration: %s\n') % (name,)
             )

tests/test-fix.t

@@ -1338,6 +1338,8 b' pattern config is missing, and that we o'
   fixer tool has no pattern configuration: nopattern
   $ cat foo bar
   foobar (no-eol)
+  $ hg fix --debug --working-dir --config "fix.nocommand:pattern=foo.bar"
+  fixer tool has no command configuration: nocommand
 
   $ cd ..