gc: Fix hang during repack in partial clones

Add `--missing=allow-promisor` to `git rev-list` calls in
`repack_projects`. This prevents Git from auto-fetching missing objects
from the promisor remote, which can cause stalls due to sequential
network requests.

Also add a Git version check to ensure Git is at least 2.17.0 before
running `--repack`, as `--missing=allow-promisor` was introduced in that
version.

Bug: 500133631
Change-Id: I2dcf9b46fac4c6a53a3c2a46f06f61d6aec40f2f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/570361
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Sam Saccone <samccone@google.com>

subcmds/gc.py

@@ -16,6 +16,7 @@ import os
 from typing import List, Set
 
 from command import Command
+from git_command import git_require
 from git_command import GitCommand
 import platform_utils
 from progress import Progress
@@ -204,6 +205,7 @@ class Gc(Command):
                 [
                     "rev-list",
                     "--objects",
+                    "--missing=allow-promisor",
                     f"--remotes={project.remote.name}",
                     "--filter=blob:none",
                     "--tags",
@@ -215,7 +217,12 @@ class Gc(Command):
             # Get all local objects and pack them.
             local_head_objects_cmd = GitCommand(
                 project,
-                ["rev-list", "--objects", "HEAD^{tree}"],
+                [
+                    "rev-list",
+                    "--objects",
+                    "--missing=allow-promisor",
+                    "HEAD^{tree}",
+                ],
                 capture_stdout=True,
                 verify_command=True,
             )
@@ -224,6 +231,7 @@ class Gc(Command):
                 [
                     "rev-list",
                     "--objects",
+                    "--missing=allow-promisor",
                     "--all",
                     "--reflog",
                     "--indexed-objects",
@@ -297,7 +305,8 @@ class Gc(Command):
         if ret != 0:
             return ret
 
-        if not opt.repack:
-            return
+        if opt.repack:
+            git_require((2, 17, 0), fail=True, msg="--repack")
+            ret = self.repack_projects(projects, opt)
 
-        return self.repack_projects(projects, opt)
+        return ret