Backport #39781 by @SudhanshuMatrix
## What
`parseGitDiffTreeLine` (`services/gitdiff/git_diff_tree.go`) read
`BasePath`/`HeadPath` straight from `git diff-tree --raw` output without
undoing git's C-quoting, unlike its sibling `parseLsTreeLine`
(`modules/git/parse.go`), which already does this with
`strconv.Unquote`.
This fix applies the same unquoting to both paths in the diff-tree
parser.
## Why
With the default `core.quotePath=true`, git C-quotes **any path
containing a non-ASCII byte** — not just tabs/newlines/backslashes. That
means any file with a CJK, Cyrillic, accented-Latin, or emoji name gets
octal-escaped, e.g.:
```
:100644 100644 b6fc4c6... 04fea06... M "\346\227\245\346\234\254\350\252\236.txt"
```
(`日本語.txt`, confirmed against real `git diff-tree --raw` output)
`GetDiffTree` backs the file-tree sidebar on the commit, compare, and
pull request diff pages (`routers/web/repo/commit.go`, `compare.go`,
`pull.go`). Without unquoting, any such file shows its mangled, escaped
name instead of the real one, and the wrong path can throw off
jump-to-file and viewed-file-state behavior that key off the path.
Co-authored-by: Shudhanshu Singh <sudhanshuwriterblc@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
