gc: fix untargeted projects being deleted

`delete_unused_projects` needs a full list of active projects to figure
out which orphaned .git dirs need to be deleted. Otherwise it thinks
that only the projects specified in args are active.

Bug: 447626164
Change-Id: I02beebf6a01c77742a8db78221452d71cd78ea73
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/550061
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

subcmds/gc.py

@@ -284,7 +284,16 @@ class Gc(Command):
             args, all_manifests=not opt.this_manifest_only
         )
 
-        ret = self.delete_unused_projects(projects, opt)
+        # If the user specified projects, fetch the global list separately
+        # to avoid deleting untargeted projects.
+        if args:
+            all_projects = self.GetProjects(
+                [], all_manifests=not opt.this_manifest_only
+            )
+        else:
+            all_projects = projects
+
+        ret = self.delete_unused_projects(all_projects, opt)
         if ret != 0:
             return ret