patching: rewrite: fully stabilize patch index stanzas as well as From lines

- `git format-patch --zero-commit` doesn't affect `index xxx...yyy` lines, only `From: `
  - so use the _classy_ "use a regex with a callback" solution as git format-patch doesn't offer one
- this will make _all_ patches change when rewritten, but hopefully _for the last time_ !
- we need to preserve `index 000000000000..xxx` as zeros, which indicate new file creation, thus:
  - new file creations are rewritten as `index 000000000000..111111111111`
  - non-creations are rewritten as `index 111111111111..222222222222`
- this is the final version of armbian#6455

Author: rpardini

lib/tools/common/patching_utils.py

@@ -28,6 +28,19 @@
 REGEX_PATCH_FILENAMES = r"^patching file \"(.+)\""
 log: logging.Logger = logging.getLogger("patching_utils")
 
+# Magic strings and regex for rewriting patches' "index xxx....yyyy" lines
+index_zero = f"{'0' * 12}"
+index_from_zero = f"index {'0' * 12}..{'1' * 12}"
+index_not_zero = f"index {'1' * 12}..{'2' * 12}"
+index_rewrite_regexp: re.Pattern = re.compile(r"index ([0-9a-f]{12})\.\.([0-9a-f]{12})")
+
+
+# Callback used for rewriting index lines.
+def rewrite_indexes_callback(x: re.Match):  # Preserve zero from's for new file creations.
+	if x.group(1) == index_zero:
+		return index_from_zero
+	return index_not_zero
+
 
 class PatchRootDir:
 	def __init__(self, abs_dir, root_type, patch_type, root_dir):
@@ -808,7 +821,7 @@ def export_commit_as_patch(repo: git.Repo, commit: str):
 		'--zero-commit',  # do not use the git revision, instead 000000...0000
 		'--stat=120',  # 'wider' stat output; default is 80
 		'--stat-graph-width=10',  # shorten the diffgraph graph part, it's too long
-        '--abbrev=12', # force index length to 12
+		'--abbrev=12',  # force index length to 12 - essential for the regex below to work
 		"-1", "--stdout", commit
 	],
 		cwd=repo.working_tree_dir,
@@ -823,7 +836,17 @@ def export_commit_as_patch(repo: git.Repo, commit: str):
 		raise Exception(f"Failed to export commit {commit} to patch: {stderr_output}")
 	if stdout_output == "":
 		raise Exception(f"Failed to export commit {commit} to patch: no output")
-	return stdout_output
+
+	# Now, massage the output. We don't want the "index 08c33ec7e9f1..528741fcc0ec 100644" lines changing every time.
+	# We do need to preserve "0000000000.." ones as that indicates new file creation.
+	# Use a regular expression and a callback to decide. Check the top of this file for the regex and callback.
+	rewritten_indexes = re.sub(index_rewrite_regexp, rewrite_indexes_callback, stdout_output)
+
+	# If rewritten is same as original this didn't work, surely.
+	if rewritten_indexes == stdout_output:
+		raise Exception(f"Failed to rewrite indexes in patch output: {stdout_output}")
+
+	return rewritten_indexes
 
 
 # Hack