support git packed refs in version generator

(cherry picked from commit 77cf65804c732431574a99063f2132567d54a6be)

methods.py

@@ -107,9 +107,21 @@ def update_version(module_version_string=""):
     if os.path.isfile(os.path.join(gitfolder, "HEAD")):
         head = open_utf8(os.path.join(gitfolder, "HEAD"), "r").readline().strip()
         if head.startswith("ref: "):
-            head = os.path.join(gitfolder, head[5:])
+            ref = head[5:]
+            head = os.path.join(gitfolder, ref)
+            packedrefs = os.path.join(gitfolder, "packed-refs")
             if os.path.isfile(head):
                 githash = open(head, "r").readline().strip()
+            elif os.path.isfile(packedrefs):
+                # Git may pack refs into a single file. This code searches .git/packed-refs file for the current ref's hash.
+                # https://mirrors.edge.kernel.org/pub/software/scm/git/docs/git-pack-refs.html
+                for line in open(packedrefs, "r").read().splitlines():
+                    if line.startswith("#"):
+                        continue
+                    (line_hash, line_ref) = line.split(" ")
+                    if ref == line_ref:
+                        githash = line_hash
+                        break
         else:
             githash = head