Fix incremental syncs for prjs with submodules

When performing an incremental sync (re-running repo init with an
updated manifest revision) with --fetch-submodules or sync-s=true,
there is an attempt to get a list of all projects (including
submodules) before projects are actually fetched. However, we can
only list submodules of a project if we have already fetched its
revision. Instead of throwing an error when we don't have the
revision, assume there are no submodules for that project. In the
sync cmd, we already update the list of projects to include
submodules after fetching superprojects.

Change-Id: I48bc68c48b5b10117356b18f5375d17f9a89ec05
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/439761
Commit-Queue: Kaushik Lingarkar <kaushik.lingarkar@linaro.org>
Tested-by: Kaushik Lingarkar <kaushik.lingarkar@linaro.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Nasser Grainawi <nasser.grainawi@linaro.org>

project.py

@@ -2296,7 +2296,9 @@ class Project:
 
         try:
             rev = self.GetRevisionId()
-        except GitError:
+        except (GitError, ManifestInvalidRevisionError):
+            # The git repo may be outdated (i.e. not fetched yet) and querying
+            # its submodules using the revision may not work; so return here.
             return []
         return get_submodules(self.gitdir, rev)