git: the git_diff tools return undecodable text for non-UTF-8 file content #5193

Problem

git_diff_unstaged, git_diff_staged and git_diff in src/git/src/mcp_server_git/server.py return GitPython's diff output unchanged. GitPython decodes with surrogateescape, so a diff of a non-UTF-8 text file (Latin-1, for example) contains lone surrogates, which cannot be encoded as UTF-8 when the tool result is serialized. Reproduced on v2/main. git_show already decodes with replacement characters.

Expected

The diff tools replace undecodable bytes with U+FFFD, as git_show does, and return a normal result.

Prior work

• fix(git): handle non-UTF-8 bytes in diff output #4848 by @BlueX888

The PR(s) above targeted the pre-refactor code and are closed as part of the backlog triage in #4875; this issue is where the work continues, with the PR(s) as a prototype. Thank you to their authors.