commitextras: make sure keys are not empty

Thanks to Yuya for catching this.

hgext/commitextras.py

@@ -54,6 +54,9 b' def _commit(orig, ui, repo, *pats, **opt'
                                 "KEY=VALUE format")
                         raise error.Abort(msg % raw)
                     k, v = raw.split('=', 1)
+                    if not k:
+                        msg = _("unable to parse '%s', keys can't be empty")
+                        raise error.Abort(msg % raw)
                     if re.search('[^\w-]', k):
                         msg = _("keys can only contain ascii letters, digits,"
                                 " '_' and '-'")

tests/test-commit.t

@@ -138,6 +138,9 b' Using the advanced --extra flag'
   $ hg commit -m "special chars in extra" --extra id@phab=214
   abort: keys can only contain ascii letters, digits, '_' and '-'
   [255]
+  $ hg commit -m "empty key" --extra =value
+  abort: unable to parse '=value', keys can't be empty
+  [255]
   $ hg commit -m "adding extras" --extra sourcehash=foo --extra oldhash=bar
   $ hg log -r . -T '{extras % "{extra}\n"}'
   branch=default