forall: add an --ignore-missing option

In CrOS, our infra has to deal with partial checkouts constantly
(for a variety of reasons).  To help reset back to a good state,
we run git commands via `repo forall`, but don't care about the
missing checkouts.  Add a flag so we can disambiguate between
missing repos and failing git subcommands.

Bug: https://crbug.com/1013377
Bug: https://crbug.com/1013623
Change-Id: Ie3498c6d111276c60d2ecedbba21bfa778588d50
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/241935
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

subcmds/forall.py

@@ -139,6 +139,9 @@ without iterating through the remaining projects.
     p.add_option('-e', '--abort-on-errors',
                  dest='abort_on_errors', action='store_true',
                  help='Abort if a command exits unsuccessfully')
+    p.add_option('--ignore-missing', action='store_true',
+                 help='Silently skip & do not exit non-zero due missing '
+                      'checkouts')
 
     g = p.add_option_group('Output')
     g.add_option('-p',
@@ -323,6 +326,10 @@ def DoWork(project, mirror, opt, cmd, shell, cnt, config):
     cwd = project['worktree']
 
   if not os.path.exists(cwd):
+    # Allow the user to silently ignore missing checkouts so they can run on
+    # partial checkouts (good for infra recovery tools).
+    if opt.ignore_missing:
+      return 0
     if ((opt.project_header and opt.verbose)
         or not opt.project_header):
       print('skipping %s/' % project['relpath'], file=sys.stderr)