Add previous_got_revision and previous_got_revision_cp to output

As chrome browser infra works to improve CQ times, we would like to
easily glance at builds and see how many new commits were checked out.
Currently you have to open up bot_update stdout to see the previous
HEAD revision and go from there. Having this conveniently outputted
would be very convenient.

If the bot has previously checked out the repo, output which revision
and commit position were checked out.

example LED build showing new output: https://ci.chromium.org/swarming/task/51592e46d6a70e10?server=chromium-swarm.appspot.com

Change-Id: Ib8a6f152a593446ce4082477bc9ada44f1fd1fc3
Bug: 1170568
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2644829Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -839,7 +839,8 @@ def get_commit_position(git_path, revision='HEAD'):
   return None
 
 
-def parse_got_revision(gclient_output, got_revision_mapping):
+def parse_got_revision(gclient_output, got_revision_mapping,
+                       prev_checkout_info):
   """Translate git gclient revision mapping to build properties."""
   properties = {}
   solutions_output = {
@@ -862,9 +863,16 @@ def parse_got_revision(gclient_output, got_revision_mapping):
       revision = git('rev-parse', 'HEAD', cwd=dir_name).strip()
       commit_position = get_commit_position(dir_name)
 
+    prev_revision, prev_cp = prev_checkout_info.get(dir_name.rstrip('/'),
+                                                    (None, None))
+
     properties[property_name] = revision
+    if prev_revision:
+      properties['prev_%s' % property_name] = prev_revision
     if commit_position:
       properties['%s_cp' % property_name] = commit_position
+    if prev_cp:
+      properties['prev_%s_cp' % property_name] = prev_cp
 
   return properties
 
@@ -1105,6 +1113,16 @@ def prepare(options, git_slns, active):
   print('Fetching Git checkout at %s@%s' % (first_sln, revisions[first_sln]))
   return revisions, step_text
 
+def current_checkout_info(solutions):
+  r = {}
+  build_dir = os.getcwd()
+
+  for sln in solutions:
+    sln_dir = path.join(build_dir, sln['name'])
+    if path.isdir(sln_dir) and not is_broken_repo_dir(sln_dir):
+      r[sln['name']] = (git('rev-parse', 'HEAD', cwd=sln_dir).strip(),
+                        get_commit_position(sln_dir))
+  return r
 
 def checkout(options, git_slns, specs, revisions, step_text):
   print('Using Python version: %s' % (sys.version,))
@@ -1128,6 +1146,8 @@ def checkout(options, git_slns, specs, revisions, step_text):
     # create file, no content
     pass
 
+  prev_checkout_info = current_checkout_info(git_slns)
+
   should_delete_dirty_file = False
 
   try:
@@ -1201,7 +1221,8 @@ def checkout(options, git_slns, specs, revisions, step_text):
   if not revision_mapping:
     revision_mapping['got_revision'] = first_sln
 
-  got_revisions = parse_got_revision(gclient_output, revision_mapping)
+  got_revisions = parse_got_revision(gclient_output, revision_mapping,
+                                     prev_checkout_info)
 
   if not got_revisions:
     # TODO(hinoka): We should probably bail out here, but in the interest