bot_update: only fetch explicit refs for the main solution.

Bug: 865882
Change-Id: I27bfbeaf7395a05ce660e9e961550233461acb3d
Reviewed-on: https://chromium-review.googlesource.com/1175857
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>

recipes/README.recipes.md

@@ -53,7 +53,7 @@ Wrapper for easy calling of bot_update.
 
 — **def [apply\_gerrit\_ref](/recipes/recipe_modules/bot_update/api.py#52)(self, root, gerrit_no_reset=False, gerrit_no_rebase_patch_ref=False, gerrit_repo=None, gerrit_ref=None, step_name='apply_gerrit', \*\*kwargs):**
 
-— **def [deapply\_patch](/recipes/recipe_modules/bot_update/api.py#408)(self, bot_update_step):**
+— **def [deapply\_patch](/recipes/recipe_modules/bot_update/api.py#413)(self, bot_update_step):**
 
 Deapplies a patch, taking care of DEPS and solution revisions properly.
     
@@ -69,7 +69,7 @@ Args:
   manifest_name: The name of the manifest to upload to LogDog.  This must
     be unique for the whole build.
 
-— **def [get\_project\_revision\_properties](/recipes/recipe_modules/bot_update/api.py#385)(self, project_name, gclient_config=None):**
+— **def [get\_project\_revision\_properties](/recipes/recipe_modules/bot_update/api.py#390)(self, project_name, gclient_config=None):**
 
 Returns all property names used for storing the checked-out revision of
 a given project.

recipes/recipe_modules/bot_update/api.py

@@ -184,7 +184,12 @@ class BotUpdateApi(recipe_api.RecipeApi):
           fixed_revision = self._destination_branch(cfg, name)
         # If we're syncing to a ref, we want to make sure it exists before
         # trying to check it out.
-        if fixed_revision.startswith('refs/'):
+        if (fixed_revision.startswith('refs/') and
+            # TODO(crbug.com/874501): fetching additional refs is currently
+            # only supported for the root solution. We should investigate
+            # supporting it for other dependencies.
+            cfg.solutions and
+            cfg.solutions[0].name == name):
           # Handle the "ref:revision" syntax, e.g.
           # refs/branch-heads/4.2:deadbeef
           refs.append(fixed_revision.split(':')[0])