repo download: add --cherry-pick option

default option uses git checkout, and thus overwrite the previous
checkouts.  this is a problem for automated builds of several
changesets in the same project for daily builds of pending submission

You can now use:
repo download [--cherry-pick|-c] project changeid/patchnumber

This will parse the manifest, cd to the corresponding project
download the changes to FETCH_HEAD and cherry-pick the result.

This is useful to automate cherry-picking of a patch
in the context of build automation, and commit gating (e.g. buildbot)

Change-Id: Ib638afd87677f1be197afb7b0f73c70fb98909fe
Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>

project.py

@@ -1637,6 +1637,14 @@ class Project(object):
       if self._allrefs:
         raise GitError('%s checkout %s ' % (self.name, rev))
 
+  def _CherryPick(self, rev, quiet=False):
+    cmd = ['cherry-pick']
+    cmd.append(rev)
+    cmd.append('--')
+    if GitCommand(self, cmd).Wait() != 0:
+      if self._allrefs:
+        raise GitError('%s cherry-pick %s ' % (self.name, rev))
+
   def _ResetHard(self, rev, quiet=True):
     cmd = ['reset', '--hard']
     if quiet: