subcmds: reduce multiprocessing serialization overhead

Follow the same approach as 39ffd9977e to reduce serialization overhead.

Below benchmarks are tested with 2.7k projects on my workstation
(warm cache). git tracing is disabled for benchmark.

(seconds)              | v2.48 | v2.48 | this CL | this CL
	               |       |  -j32 |         |    -j32
-----------------------------------------------------------
with clean tree state:
branches (none)        |   5.6 |   5.9 |    1.0  |    0.9
status (clean)         |  21.3 |   9.4 |   19.4  |    4.7
diff (none)            |   7.6 |   7.2 |    5.7  |    2.2
prune (none)           |   5.7 |   6.1 |    1.3  |    1.2
abandon (none)         |  19.4 |  18.6 |    0.9  |    0.8
upload (none)          |  19.7 |  18.7 |    0.9  |    0.8
forall -c true         |   7.5 |   7.6 |    0.6  |    0.6
forall -c "git log -1" |  11.3 |  11.1 |    0.6  |    0.6

with branches:
start BRANCH --all     |  21.9 |  20.3 |   13.6  |    2.6
checkout BRANCH        |  29.1 |  27.8 |    1.1  |    1.0
branches (2)           |  28.0 |  28.6 |    1.5  |    1.3
abandon BRANCH         |  29.2 |  27.5 |    9.7  |    2.2

Bug: b/371638995
Change-Id: I53989a3d1e43063587b3f52f852b1c2c56b49412
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/440221
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Kuang-che Wu <kcwu@google.com>
Commit-Queue: Kuang-che Wu <kcwu@google.com>

subcmds/status.py

@@ -88,7 +88,8 @@ the following meanings:
             "projects",
         )
 
-    def _StatusHelper(self, quiet, local, project):
+    @classmethod
+    def _StatusHelper(cls, quiet, local, project_idx):
         """Obtains the status for a specific project.
 
         Obtains the status for a project, redirecting the output to
@@ -99,12 +100,13 @@ the following meanings:
             local: a boolean, if True, the path is relative to the local
                 (sub)manifest.  If false, the path is relative to the outermost
                 manifest.
-            project: Project to get status of.
+            project_idx: Project index to get status of.
 
         Returns:
             The status of the project.
         """
         buf = io.StringIO()
+        project = cls.get_parallel_context()["projects"][project_idx]
         ret = project.PrintWorkTreeStatus(
             quiet=quiet, output_redir=buf, local=local
         )
@@ -143,15 +145,18 @@ the following meanings:
                     ret += 1
             return ret
 
-        counter = self.ExecuteInParallel(
-            opt.jobs,
-            functools.partial(
-                self._StatusHelper, opt.quiet, opt.this_manifest_only
-            ),
-            all_projects,
-            callback=_ProcessResults,
-            ordered=True,
-        )
+        with self.ParallelContext():
+            self.get_parallel_context()["projects"] = all_projects
+            counter = self.ExecuteInParallel(
+                opt.jobs,
+                functools.partial(
+                    self._StatusHelper, opt.quiet, opt.this_manifest_only
+                ),
+                range(len(all_projects)),
+                callback=_ProcessResults,
+                ordered=True,
+                chunksize=1,
+            )
 
         if not opt.quiet and len(all_projects) == counter:
             print("nothing to commit (working directory clean)")