Stop parsing when we reach an unknown word

When parsing a @bors command, stop parsing when we reach any unknown
word. This enables commands like

    @bors retry (yielding priority to the rollup)

to include a description of why we retried (which gets put in the retry
log) without also setting `rollup`.

The only tricky bit to this is a command like

    @bors r=person 0123456789abcdef

where the commit sha is part of the `r=` command, and we need to parse
it that way.

homu/parse_issue_comment.py

@@ -158,13 +158,21 @@ def parse_issue_comment(username, body, sha, botname, hooks=[]):
     if words[1:] == ["are", "you", "still", "there?"]:
         commands.append(IssueCommentCommand.ping('portal'))
 
-    for i, word in reversed(list(enumerate(words))):
-        found = True
+    for i, word in enumerate(words):
+        if word is None:
+            # We already parsed the next word, and we set it to an empty string
+            # to signify that we did.
+            continue
+
+        if word == '@' + botname:
+            continue
+
         if word == 'r+' or word.startswith('r='):
             approved_sha = sha
 
             if i + 1 < len(words) and is_sha(words[i + 1]):
                 approved_sha = words[i + 1]
+                words[i + 1] = None
 
             approver = word[len('r='):] if word.startswith('r=') else username
 
@@ -238,9 +246,7 @@ def parse_issue_comment(username, body, sha, botname, hooks=[]):
             commands.append(IssueCommentCommand.hook(hook_name, hook_extra))
 
         else:
-            found = False
-
-        if found:
-            words[i] = ''
+            # First time we reach an unknown word, stop parsing.
+            break
 
     return commands

homu/tests/test_parse_issue_comment.py

@@ -465,6 +465,31 @@ def test_multiple_hooks():
     assert thirdhook_commands[0].hook_extra is None
 
 
+def test_parse_up_to_first_unknown_word():
+    """
+    Test that when parsing, once we arrive at an unknown word, we stop parsing
+    """
+
+    author = "jack"
+    body = """
+    @bors retry -- yielding priority to the rollup
+    """
+    commands = parse_issue_comment(author, body, commit, "bors")
+
+    assert len(commands) == 1
+    command = commands[0]
+    assert command.action == 'retry'
+
+    body = """
+    @bors retry (yielding priority to the rollup)
+    """
+    commands = parse_issue_comment(author, body, commit, "bors")
+
+    assert len(commands) == 1
+    command = commands[0]
+    assert command.action == 'retry'
+
+
 def test_ignore_commands_before_bors_line():
     """
     Test that when command-like statements appear before the @bors part,