[bot_update] add ignore_input_commit parameter

CrOS chromium-pfq builder uses bot_update, but checks out Chromium on its own,
effectively ignoring input commit in bot_update.
To resolve a P0, add ignore_input_commit parameter.
Long term, bot_update API must be redesigned.

TBR=tandrii@chromium.org
Bug: 882094
Change-Id: I4f43348e3d5f81bf603dd2c110829bce58acad16
Reviewed-on: https://chromium-review.googlesource.com/1216787Reviewed-by: Lann Martin <lannm@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>

recipes/recipe_modules/bot_update/api.py

@@ -80,7 +80,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
                       patchset=None, gerrit_no_reset=False,
                       gerrit_no_rebase_patch_ref=False,
                       disable_syntax_validation=False, manifest_name=None,
-                      patch_refs=None,
+                      patch_refs=None, ignore_input_commit=False,
                       **kwargs):
     """
     Args:
@@ -153,6 +153,9 @@ class BotUpdateApi(recipe_api.RecipeApi):
       if solution.revision:
         revisions[solution.name] = solution.revision
 
+    # HACK: ensure_checkout API must be redesigned so that we don't pass such
+    # parameters. Existing semantics is too opiniated.
+    if not ignore_input_commit:
       # Apply input gitiles_commit, if any.
       input_commit = self.m.buildbucket.build.input.gitiles_commit
       if input_commit.id or input_commit.ref:
@@ -160,8 +163,8 @@ class BotUpdateApi(recipe_api.RecipeApi):
         # Note: this is not entirely correct. build.input.gitiles_commit
         # definition says "The Gitiles commit to run against.".
         # However, here we ignore it if the config specified a revision.
-      # This is necessary because existing builders rely on this behavior, e.g.
-      # they want to force refs/heads/master at the config level.
+        # This is necessary because existing builders rely on this behavior,
+        # e.g. they want to force refs/heads/master at the config level.
         revisions[repo_path] = (
             revisions.get(repo_path) or input_commit.id or input_commit.ref)