status: add -q/--quiet option

The --quiet option reduces the output to just
a list of projects with modified workspaces (and
orphans if -o is specified)

A common use case is when performing a full-workspace
merge.  The integrator will kick-off a merge via:

    repo forall -c git merge <some tag>

And then produce a short list of conflicted projects via:

    repo status -q

The integrator can then iteratively fix and clean up all conficted
components.  The merge is complete when:

    repo status -q

    returns no output.

Change-Id: Ibbba8713eac35befd8287c95948874e23fd5c7e2

project.py

@@ -911,11 +911,13 @@ class Project(object):
     else:
       return False
 
-  def PrintWorkTreeStatus(self, output_redir=None):
+  def PrintWorkTreeStatus(self, output_redir=None, quiet=False):
     """Prints the status of the repository to stdout.
 
     Args:
       output: If specified, redirect the output to this object.
+      quiet:  If True then only print the project name.  Do not print
+              the modified files, branch name, etc.
     """
     if not os.path.isdir(self.worktree):
       if output_redir is None:
@@ -941,6 +943,10 @@ class Project(object):
       out.redirect(output_redir)
     out.project('project %-40s', self.relpath + '/ ')
 
+    if quiet:
+      out.nl()
+      return 'DIRTY'
+
     branch = self.CurrentBranch
     if branch is None:
       out.nobranch('(*** NO BRANCH ***)')