Fix diff parsing to support mnemonicPrefix configuration

[MirrorDNA-Reflection-Protocol]

Summary

Fixes #2013

When diff.mnemonicPrefix=true is set in git config, git uses different prefixes for diff paths instead of the standard a/ and b/:

• c/ for commit
• w/ for worktree
• i/ for index
• o/ for object
• h/ for HEAD

Previously, the diff regex and decode_path() function only accepted a/ and b/ prefixes, causing create_patch=True diffs to fail parsing when users had this config enabled.

Reproduction

As described in #2013:

# With ~/.gitconfig containing:
# [diff]
#     mnemonicPrefix = true

repo = git.Repo('.')
ancestor_ref = repo.merge_base(repo.head, repo.refs['main'])
diff = ancestor_ref[0].diff(None, create_patch=True)  # Would fail to parse

Changes

• Update re_header regex to accept [abciwoh]/ prefixes instead of just [ab]/
• Update decode_path() assertion to accept all valid mnemonic prefixes
• Add test case for mnemonicPrefix-style diffs

Testing

• New test test_diff_mnemonic_prefix verifies parsing works with c/ and w/ prefixes
• All existing diff tests pass (except one pre-existing failure on main unrelated to this change)

References

• Git documentation: https://git-scm.com/docs/git-config#Documentation/git-config.txt-diffmnemonicPrefix
• Issue: gitconfig: mnemonicPrefix conflict with create_patch #2013