sync: keep generated jobs-checkout help host-independent

The default shown for `--jobs-checkout` comes from `DEFAULT_LOCAL_JOBS`,
which depends on the host CPU count. As a result, regenerating the sync
and smartsync manpages on a different machine can change their output
even though repo's behavior has not changed.

Use shared CPU-based help formatting for `--jobs` and `--jobs-checkout`.
Interactive help still shows the actual default, while generated
manpages describe it without including a host-specific value.
Runtime job selection is unchanged.

Add coverage for both help modes and regenerate the affected manpages.

Change-Id: Ia60e6fcc17b6f4618c52a76217c128eddead8848
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/625381
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Victor Pushkarev <corvinus.v@gmail.com>
Tested-by: Victor Pushkarev <corvinus.v@gmail.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

subcmds/sync.py

@@ -534,6 +534,8 @@ later is required to fix a server side protocol bug.
             help="number of network jobs to run in parallel (defaults to "
             "--jobs or 1). Ignored unless --no-interleaved is set",
         )
+
+        jobs_checkout_default = self._GetHelpForCpuJobCount(DEFAULT_LOCAL_JOBS)
         p.add_option(
             "--jobs-checkout",
             default=None,
@@ -541,7 +543,7 @@ later is required to fix a server side protocol bug.
             metavar="JOBS",
             help=(
                 "number of local checkout jobs to run in parallel (defaults "
-                f"to --jobs or {DEFAULT_LOCAL_JOBS}). Ignored unless "
+                f"to --jobs or {jobs_checkout_default}). Ignored unless "
                 "--no-interleaved is set"
             ),
         )