Add .gitattribute assisted language detection to blame, diff and render (#17590)

Use check attribute code to check the assigned language of a file and send that in to
chroma as a hint for the language of the file.

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/web/repo/view.go

@@ -502,7 +502,33 @@ func renderFile(ctx *context.Context, entry *git.TreeEntry, treeLink, rawLink st
 			lineNums := linesBytesCount(buf)
 			ctx.Data["NumLines"] = strconv.Itoa(lineNums)
 			ctx.Data["NumLinesSet"] = true
-			ctx.Data["FileContent"] = highlight.File(lineNums, blob.Name(), buf)
+
+			language := ""
+
+			indexFilename, worktree, deleteTemporaryFile, err := ctx.Repo.GitRepo.ReadTreeToTemporaryIndex(ctx.Repo.CommitID)
+			if err == nil {
+				defer deleteTemporaryFile()
+
+				filename2attribute2info, err := ctx.Repo.GitRepo.CheckAttribute(git.CheckAttributeOpts{
+					CachedOnly: true,
+					Attributes: []string{"linguist-language", "gitlab-language"},
+					Filenames:  []string{ctx.Repo.TreePath},
+					IndexFile:  indexFilename,
+					WorkTree:   worktree,
+				})
+				if err != nil {
+					log.Error("Unable to load attributes for %-v:%s. Error: %v", ctx.Repo.Repository, ctx.Repo.TreePath, err)
+				}
+
+				language = filename2attribute2info[ctx.Repo.TreePath]["linguist-language"]
+				if language == "" || language == "unspecified" {
+					language = filename2attribute2info[ctx.Repo.TreePath]["gitlab-language"]
+				}
+				if language == "unspecified" {
+					language = ""
+				}
+			}
+			ctx.Data["FileContent"] = highlight.File(lineNums, blob.Name(), language, buf)
 		}
 		if !isLFSFile {
 			if ctx.Repo.CanEnableEditor() {