Ensure single trailing newline in source files

Reduce future code churn by gating source files with a test that ensures
tracked text files in the repository end with a single trailing newline
(UNIX convention).

Alternatives to enforce this without a test:

An .editorconfig file at the root of the repository makes most editors
automatically insert a final newline and trim trailing whitespace on
save, preventing the problem at the source:

    root = true

    [*]
    insert_final_newline = true
    trim_trailing_whitespace = true
    end_of_line = lf
    charset = utf-8

A .gitattributes file makes Git itself flag whitespace issues in diffs,
on apply, and on merge:

    * text=auto eol=lf whitespace=blank-at-eol,blank-at-eof

Both files support per-path exceptions. In .editorconfig, a section like
[*.png] can override any setting. In .gitattributes, paths can opt out
with -whitespace or be marked as binary (e.g. "*.png binary").

Author: rhcarvalho

.github/ISSUE_TEMPLATE/bug_report.md

@@ -24,3 +24,6 @@ https://github.com/wojtekmach/mix_install_examples/blob/main/phoenix.exs
 
 ### Expected behavior
 
+<!--
+Describe the expected behaviour.
+-->

test/phoenix/source_files_test.exs

@@ -0,0 +1,36 @@
+defmodule Phoenix.SourceFilesTest do
+  use ExUnit.Case, async: true
+
+  @repo_root Path.expand("../..", __DIR__)
+  @git_repo? File.dir?(Path.join(@repo_root, ".git")) and System.find_executable("git") != nil
+  @excluded_exts ~w(.br .foo .gz .ico .lock .map .pem .png)
+  @excluded_files ~w(test/fixtures/hello.txt)
+
+  @tag skip: if(not @git_repo?, do: "git or .git repository not available")
+  test "all tracked text files in the repository end with a single newline" do
+    {output, 0} = System.cmd("git", ["ls-files", "-z"], cd: @repo_root)
+
+    target_files =
+      output
+      |> String.split("\0", trim: true)
+      |> Enum.reject(fn file ->
+        file in @excluded_files or Enum.any?(@excluded_exts, &String.ends_with?(file, &1))
+      end)
+      |> Enum.map(&Path.expand(&1, @repo_root))
+      |> Enum.filter(&File.regular?/1)
+
+    assert target_files != [], "No tracked files found in the repository"
+
+    offending =
+      target_files
+      |> Enum.reject(fn file ->
+        content = File.read!(file)
+        content == "" or content =~ ~r/\S\n\z/
+      end)
+      |> Enum.map(&Path.relative_to(&1, @repo_root))
+
+    assert offending == [],
+           "Expected the following files to end with a single trailing newline:\n\n" <>
+             Enum.map_join(offending, "\n", &"  - #{&1}")
+  end
+end