fix(stage): staging of files with no nl at eof

Previously when diffing two files where one did not have a newline at
the end of the file, gitsigns was unable to stage such differences since
this information was not captured during the diff stage.

If you run `vim.diff('a', 'a\n')` you get the result:

    @@ -1 +1 @@
    -a
    \ No newline at end of file
    +a

However if you run `vim.diff('a', 'a\n', {result_type='indices'})` you
get:

    { {1, 1, 1, 1} }

And since Gitsigns tracks changes as a list of text lines, the
information about a missing newline at the end of the file is not
correctly tracked.

The main consequence of this is that staging hunks which contain these
lines would result in an error as the generated patch would not apply
since it was missing "\ No newline at end of file".

To fix this, the internal hunk object now tracks this end of file
information and patches are now generated correctly.

lua/gitsigns/diff_int.lua

@@ -102,11 +102,17 @@ function M.run_diff(fa, fb, linematch)
       for i = rs, rs + rc - 1 do
         hunk.removed.lines[#hunk.removed.lines + 1] = fa[i] or ''
       end
+      if rs + rc >= #fa and fa[#fa] ~= '' then
+        hunk.removed.no_nl_at_eof = true
+      end
     end
     if ac > 0 then
       for i = as, as + ac - 1 do
         hunk.added.lines[#hunk.added.lines + 1] = fb[i] or ''
       end
+      if as + ac >= #fb and fb[#fb] ~= '' then
+        hunk.added.no_nl_at_eof = true
+      end
     end
     hunks[#hunks + 1] = hunk
   end

lua/gitsigns/hunks.lua

@@ -11,6 +11,7 @@ local min, max = math.min, math.max
 --- @field start integer
 --- @field count integer
 --- @field lines string[]
+--- @field no_nl_at_eof? true
 
 --- @class (exact) Gitsigns.Hunk.Hunk
 --- @field type Gitsigns.Hunk.Type
@@ -265,10 +266,19 @@ function M.create_patch(relpath, hunks, mode_bits, invert)
     for _, l in ipairs(pre_lines) do
       results[#results + 1] = '-' .. l
     end
+
+    if process_hunk.removed.no_nl_at_eof then
+      results[#results + 1] = '\\ No newline at end of file'
+    end
+
     for _, l in ipairs(now_lines) do
       results[#results + 1] = '+' .. l
     end
 
+    if process_hunk.added.no_nl_at_eof then
+      results[#results + 1] = '\\ No newline at end of file'
+    end
+
     process_hunk.removed.start = start + offset
     offset = offset + (now_count - pre_count)
   end

test/actions_spec.lua

@@ -302,4 +302,23 @@ describe('actions', function()
     command('Gitsigns prev_hunk')
     check_cursor({ 4, 0 })
   end)
+
+  it('can stage hunks with no NL at EOF', function()
+    setup_test_repo()
+    local newfile = helpers.newfile
+    exec_lua([[vim.g.editorconfig = false]])
+    system("printf 'This is a file with no nl at eof' > "..newfile)
+    helpers.gitm({
+      { 'add', newfile },
+      { 'commit', '-m', 'commit on main' },
+    })
+
+    edit(newfile)
+    check({ status = { head = 'master', added = 0, changed = 0, removed = 0 } })
+    feed('x')
+    check({ status = { head = 'master', added = 0, changed = 1, removed = 0 } })
+    command('Gitsigns stage_hunk')
+    check({ status = { head = 'master', added = 0, changed = 0, removed = 0 } })
+  end)
+
 end)