the regex:

is not able to match the diff generated if this flag is enabled in ~/.gitconfig:

[diff]
        mnemonicPrefix = true

because it is expecting [ab]/ as folder prefix for the diff but with mnemonicPrefix enabled it become [wc]/

Diff without mnemonicPrefix

diff --git a/.vscode/launch.json b/.vscode/launch.json
index xxxxxxx..xxxxxxxx
--- a/.vscode/launch.json
+++ b/.vscode/launch.json

Diff with mnemonicPrefix = true

diff --git c/.vscode/launch.json w/.vscode/launch.json
index xxxxxxx..xxxxxxxx
--- c/.vscode/launch.json
+++ w/.vscode/launch.json

Quick Test

repo = git.Repo('.')
branch_ref = repo.head
ancestor_ref = repo.merge_base(branch_ref, repo.refs["main"])
diff = ancestor_ref[0].diff(None, create_patch=True)
print(diff)