git cl: use project~number on Gerrit for GetChangeCommit.

R=ehmaldonado, hinoka

Bug: 876910
Change-Id: I6e288283c0033fb48a0b8ece8c06f6813f9a9d8a
Reviewed-on: https://chromium-review.googlesource.com/1186158
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index e3b868f..3eebd26 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2742,13 +2742,13 @@
     return data
 
   def _GetChangeCommit(self):
-    issue = self.GetIssue()
-    assert issue, 'issue is required to query Gerrit'
+    assert self.GetIssue(), 'issue must be set to query Gerrit'
     try:
-      data = gerrit_util.GetChangeCommit(self._GetGerritHost(), str(issue))
+      data = gerrit_util.GetChangeCommit(
+          self._GetGerritHost(), self._GerritChangeIdentifier())
     except gerrit_util.GerritError as e:
       if e.http_status == 404:
-        raise GerritChangeNotExists(issue, self.GetCodereviewServer())
+        raise GerritChangeNotExists(self.GetIssue(), self.GetCodereviewServer())
       raise
     return data