Describe upload --replace in upload's help text

Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/upload.py

@@ -55,6 +55,12 @@ If the --reviewers or --cc options are passed, those emails are
 added to the respective list of users, and emails are sent to any
 new users.  Users passed to --reviewers must be already registered
 with the code review system, or the upload will fail.
+
+If the --replace option is passed the user can designate which
+existing change(s) in Gerrit match up to the commits in the branch
+being uploaded.  For each matched pair of change,commit the commit
+will be added as a new patch set, completely replacing the set of
+files and description associated with the change in Gerrit.
 """
 
   def _Options(self, p):
@@ -151,7 +157,7 @@ with the code review system, or the upload will fail.
       _die("nothing uncommented for upload")
     self._UploadAndReport(todo, people)
 
-  def _ReplaceBranch(self, project):
+  def _ReplaceBranch(self, project, people):
     branch = project.CurrentBranch
     if not branch:
       print >>sys.stdout, "no branches ready for upload"
@@ -178,6 +184,7 @@ with the code review system, or the upload will fail.
     for line in script:
       m = change_re.match(line)
       if m:
+        c = m.group(1)
         f = m.group(2)
         try:
           f = full_hashes[f]
@@ -185,7 +192,11 @@ with the code review system, or the upload will fail.
           print 'fh = %s' % full_hashes
           print >>sys.stderr, "error: commit %s not found" % f
           sys.exit(1)
-        to_replace[m.group(1)] = f
+        if c in to_replace:
+          print >>sys.stderr,\
+            "error: change %s cannot accept multiple commits" % c
+          sys.exit(1)
+        to_replace[c] = f
 
     if not to_replace:
       print >>sys.stderr, "error: no replacements specified"
@@ -247,7 +258,7 @@ with the code review system, or the upload will fail.
         print >>sys.stderr, \
               'error: --replace requires exactly one project'
         sys.exit(1)
-      self._ReplaceBranch(project_list[0])
+      self._ReplaceBranch(project_list[0], people)
       return
 
     for project in project_list: