`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.
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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>