fix(gitdiff): unquote C-quoted paths from git diff-tree --raw (#39781) (#39783)

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>

services/gitdiff/git_diff_tree_test.go

@@ -324,6 +324,22 @@ func TestParseGitDiffTree(t *testing.T) {
 				},
 			},
 		},
+		{
+			// make sure our code can unquote paths no matter core.quotePath is true or false: quoted chars are like "\t" or "\{oct}"
+			Name:      "file modified with a C-quoted path",
+			GitOutput: ":100644 100644 b6fc4c620b67d95f953a5c1c1230aaab5db5a1b0 04fea06420ca60892f73becee3614f6d023a4b7f M\t" + `"\346\227\245\346\234\254\350\252\236\ttest.txt"`,
+			Expected: []*DiffTreeRecord{
+				{
+					Status:     "modified",
+					HeadPath:   "日本語\ttest.txt",
+					BasePath:   "日本語\ttest.txt",
+					HeadMode:   git.EntryModeBlob,
+					BaseMode:   git.EntryModeBlob,
+					HeadBlobID: "04fea06420ca60892f73becee3614f6d023a4b7f",
+					BaseBlobID: "b6fc4c620b67d95f953a5c1c1230aaab5db5a1b0",
+				},
+			},
+		},
 		{
 			Name:      "no changes",
 			GitOutput: ``,