Account for moved files

[samueljsb]

If files are moved, they might change ownership. This tools currently doesn't notice that: we only check files that exist in the currently checked out branch.

We should:

1. use git ls-tree instead of git ls-files to get the files that exist on the target branch;
2. use git diff to find moved files and show their codeowner changes, too.

[samueljsb]

This tool is not intended to show how ownership of files has changed when the files themselves move. So we should not show the change in ownership of moved files just because they have moved. However, there is currently a bug.

When a file is moved, a codeowner rule might be added to cover the file's new location. In that case, the tool will currently show the file as having previously been owned by nobody because the base codeowners file contained no rule for the new location. This is not correct because the file has not changed ownership! We should not report that file as having changed ownership.

e.g:

$ git show HEAD
commit b1eb7d8b673a0e4cdbcddb9327044edad306032a (HEAD -> main)
Author: Samuel Searles-Bryant <sam@samueljsb.co.uk>
Date:   Sun Apr 20 21:55:15 2025 +0100

    example commit moving a file

diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index e0e0754..a0ebd01 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -1 +1 @@
-/foo @samueljsb
+/bar @samueljsb
diff --git a/foo/example.txt b/bar/example.txt
similarity index 100%
rename from foo/example.txt
rename to bar/example.txt

$ codeowners-diff HEAD^ HEAD
1 files have changed ownership:

| file              | `HEAD^`   | `HEAD`     |
|:------------------|:----------|:-----------|
| `bar/example.txt` |           | @samueljsb |

[samueljsb]

This is not correct because the file has not changed ownership!

I no longer agree with this statement. The intent of the tool is to show the difference between what the code ownership would have been under the old version of CODEOWNERS vs the ownership under the new version of the rules. Under the old version of CODEOWNERS, the new file would have been owned by nobody, so it is correct to report that changing the code owner rules has changed who would own the file.

The intent of this tool is not to show the change in ownership of each file as it happens: it is only intended to diff the effects of changes to the rules themselves. For example, moving a file might change who owns that file, but that is not the concern of this tool.