chore: save HEAD when git-import-patches runs (#17824)

script/git-export-patches

@@ -9,6 +9,27 @@ import sys
 
 def guess_base_commit(repo):
   """Guess which commit the patches might be based on"""
+  try:
+    args = [
+      'git',
+      '-C',
+      repo,
+      'rev-parse',
+      '--verify',
+      'refs/patches/upstream-head',
+    ]
+    upstream_head = subprocess.check_output(args).strip()
+    args = [
+      'git',
+      '-C',
+      repo,
+      'rev-list',
+      '--count',
+      upstream_head + '..',
+    ]
+    num_commits = subprocess.check_output(args).strip()
+    return [upstream_head, num_commits]
+  except subprocess.CalledProcessError:
     args = [
       'git',
       '-C',

script/git-import-patches

@@ -17,6 +17,12 @@ def main(argv):
       help="use 3-way merge to resolve conflicts")
   args = parser.parse_args(argv)
 
+  # save the upstream HEAD so we can refer to it when we later export patches
+  git.update_ref(
+      repo='.',
+      ref='refs/patches/upstream-head',
+      newvalue='HEAD'
+  )
   git.am(
       repo='.',
       patch_data=patch_from_dir(args.patch_dir),

script/lib/git.py

@@ -102,6 +102,12 @@ def get_head_commit(repo):
   return subprocess.check_output(args).strip()
 
 
+def update_ref(repo, ref, newvalue):
+  args = ['git', '-C', repo, 'update-ref', ref, newvalue]
+
+  return subprocess.check_call(args)
+
+
 def reset(repo):
   args = ['git', '-C', repo, 'reset']