gc: add --keep-manifest-projects

By default, `repo gc` treats projects excluded by the configured
manifest groups as unused. This matches the active project set, but it
can remove cached Git data for projects that might be used again after
changing groups.

Add `--keep-manifest-projects` to keep Git data for projects that are
still in the manifests, even if the current groups exclude them or their
checkout is missing. Git data for projects no longer in the manifests
can still be removed.

Change-Id: I63ae18ce1c6acc2463be86f81921476b12fb27d7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/629741
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Victor Pushkarev <corvinus.v@gmail.com>
Commit-Queue: Victor Pushkarev <corvinus.v@gmail.com>

subcmds/gc.py

@@ -28,6 +28,14 @@ class Gc(Command):
     helpSummary = "Cleaning up internal repo and Git state."
     helpUsage = """
 %prog
+"""
+    helpDescription = """
+The '%prog' command removes internal Git data for projects that are no
+longer active in the current configuration.
+
+Projects excluded by the configured manifest groups are inactive by
+default. Use --keep-manifest-projects to keep Git data for projects that
+are still in the manifests.
 """
 
     def _Options(self, p):
@@ -53,6 +61,12 @@ class Gc(Command):
             help="repack all projects that use partial clone with "
             "filter=blob:none",
         )
+        p.add_option(
+            "--keep-manifest-projects",
+            default=False,
+            action="store_true",
+            help="keep Git data for projects still in the manifests",
+        )
 
     def _find_git_to_delete(
         self, to_keep: Set[str], start_dir: str
@@ -292,16 +306,21 @@ class Gc(Command):
             args, all_manifests=not opt.this_manifest_only
         )
 
-        # If the user specified projects, fetch the global list separately
-        # to avoid deleting untargeted projects.
-        if args:
-            all_projects = self.GetProjects(
+        if opt.keep_manifest_projects:
+            projects_to_keep = self.GetProjects(
+                [],
+                groups="all",
+                missing_ok=True,
+                all_manifests=not opt.this_manifest_only,
+            )
+        elif args:
+            projects_to_keep = self.GetProjects(
                 [], all_manifests=not opt.this_manifest_only
             )
         else:
-            all_projects = projects
+            projects_to_keep = projects
 
-        ret = self.delete_unused_projects(all_projects, opt)
+        ret = self.delete_unused_projects(projects_to_keep, opt)
         if ret != 0:
             return ret