fix(diff): read patch header paths the way git apply does

[Keerthana-64]

Diff._index_from_patch_format takes a file's paths from the diff --git a/<path> b/<path> line whenever a patch header has no --- and +++ lines, which is the case for binary files, empty files and mode-only changes, and it did the same for both names of a copy because the copy from line was not captured. Diff.re_header splits that line at the first space followed by a/ or b/, but git does not quote a path for containing a space, so a path containing a/ or b/ is split in the wrong place. A binary file assets b/logo.png was reported with a_path == "assets" and b_path == "logo.png b/assets b/logo.png", a copy of assets b/logo.png to copy b/logo.png came back as assets and logo.png b/copy b/logo.png, and for a quoted name such as "a/quoted\t a/tab" the cut inside the quotes made decode_path fail its prefix assertion, so diff(create_patch=True) raised AssertionError. Paths are chosen by whoever authored the commit, so a tool that uses Commit.diff(create_patch=True) to learn which files a commit touches is shown an a_path prefix of the author's choosing, does not see the real path as b_path, or gets no diff at all.

The fix captures the copy from name and uses the copy lines for the paths the way the rename lines already are. When no ---, +++, rename or copy line names the file, both names on the diff --git line are the same path, and the new _same_path_in_header splits the line the way git_header_name in git's apply.c does: at the end of a quoted first name, or at the space where the two halves agree. A line that does not split that way keeps the previous result, so the existing fixtures parse as before. The new test in test/test_diff.py fails before the change and passes after it. ruff check, ruff format --check and mypy are clean. In a full clone the suite has 3 failures (test_diff_with_staged_file, test_empty_repo and test_rebasing), and all 3 also fail without this change.