Fix angle_commit.h dependency when git packs refs

If there are too many files in .git/refs/heads/, git packs them in
.git/packed-refs.  This breaks the build as gen/angle/angle_commit.h
depends on .git/refs/heads/<branch-name>.

With this change, when gn is run and the dependency to this file is set
up, it's unpacked from .git/packed-refs if it's missing from
.git/refs/heads/.

One corner case not solved by this change is if a commit is amended with
no changes and git packs the refs (and so the
.git/refs/heads/<branch-name> file goes away), ninja doesn't rerun gn as
it doesn't believe there to be any changes, yet fails the build as the
dependency is now missing.  Running `gn gen` manually fixes the issue.
The issue is automatically fixed when switching branches.

Bug: angleproject:6131
Change-Id: Ie2404e874a5cb3374a652b93faa143acb68eaa19
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3001906
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

BUILD.gn

@@ -658,7 +658,17 @@ action("angle_commit_id") {
     branch = read_file(".git/HEAD", "string")
     result = string_split(branch)
     if (result[0] == "ref:") {
-      inputs += [ ".git/" + result[1] ]
+      _ref_file = result[1]
+
+      # If git has packed the contents of .git/refs/heads/ in .git/packed-refs,
+      # unpack the head before depending on it.
+      exec_script("src/commit_id.py",
+                  [
+                    "unpack",
+                    _ref_file,
+                  ],
+                  "")
+      inputs += [ ".git/" + _ref_file ]
     }
   }
 

src/commit_id.py

@@ -12,6 +12,8 @@ import os
 
 usage = """\
 Usage: commit_id.py check                - check if git is present
+       commit_id.py unpack <ref_file>    - check if <ref_file> exists, and if not
+                                           create it based on .git/packed-refs
        commit_id.py position             - print commit position
        commit_id.py gen <file_to_write>  - generate commit.h"""
 
@@ -25,6 +27,24 @@ def get_commit_position(cwd):
     return grab_output('git rev-list HEAD --count', cwd)
 
 
+def unpack_ref(ref_file, ref_file_full_path, packed_refs_full_path):
+
+    with open(packed_refs_full_path) as fin:
+        refs = fin.read().strip().split('\n')
+
+    # Strip comments
+    refs = [ref.split(' ') for ref in refs if ref.strip()[0] != '#']
+
+    # Parse lines (which are in the format <hash> <ref_file>) and find the input file
+    refs = [git_hash for (git_hash, file_path) in refs if file_path == ref_file]
+
+    assert (len(refs) == 1)
+    git_hash = refs[0]
+
+    with open(ref_file_full_path, 'w') as fout:
+        fout.write(git_hash + '\n')
+
+
 if len(sys.argv) < 2:
     sys.exit(usage)
 
@@ -44,6 +64,19 @@ if operation == 'check':
     else:
         print("0")
     sys.exit(0)
+elif operation == 'unpack':
+    if len(sys.argv) < 3:
+        sys.exit(usage)
+
+    ref_file = sys.argv[2]
+    ref_file_full_path = os.path.join(cwd, '.git', ref_file)
+    ref_file_exists = os.path.exists(ref_file_full_path)
+
+    if not ref_file_exists:
+        packed_refs_full_path = os.path.join(cwd, '.git', 'packed-refs')
+        unpack_ref(ref_file, ref_file_full_path, packed_refs_full_path)
+
+    sys.exit(0)
 elif operation == 'position':
     if git_dir_exists:
         print(get_commit_position(cwd))