feat: improve error handling of commit rendering

- Simplify if-else expression to `NotFoundOrServerError`.
- I cannot find an existing scenario where `Getdiff` returns an error
and where it therefore should show a 404 error in the context of
rendering a diff of a commit. So simply return it as an Internal Server
Error, this also helps with debugging if an actual error occurs
here (404 errors are only logged at the DEBUG level).
- The first change is already covered under existing testing, the second
change is not trivial to test.

routers/web/repo/commit.go

@@ -315,11 +315,7 @@ func Diff(ctx *context.Context) {
 
 	commit, err := gitRepo.GetCommit(commitID)
 	if err != nil {
-		if git.IsErrNotExist(err) {
-			ctx.NotFound("Repo.GitRepo.GetCommit", err)
-		} else {
-			ctx.ServerError("Repo.GitRepo.GetCommit", err)
-		}
+		ctx.NotFoundOrServerError("gitRepo.GetCommit", git.IsErrNotExist, err)
 		return
 	}
 	if len(commitID) != commit.ID.Type().FullLength() {
@@ -343,7 +339,7 @@ func Diff(ctx *context.Context) {
 		FileOnly:           fileOnly,
 	}, files...)
 	if err != nil {
-		ctx.NotFound("GetDiff", err)
+		ctx.ServerError("GetDiff", err)
 		return
 	}