fix(difftool): handle filenames containing spaces #39740

Problem:
Using the `DiffTool` plugin (e.g. through `nvim -d ...` or `:DiffTool
<file1> <file2>` fails if a space is in one of the paths. This occurs
because the `diff` wraps the paths with quotes (`'`) if space
characters are present, which the line diff regex fails to parse.

Solution:
Update regex to handle quoted paths by matching the string within the
quotes, if it exists.

(cherry picked from commit 1e09b020e5)

runtime/pack/dist/opt/nvim.difftool/lua/difftool.lua

@@ -123,7 +123,7 @@ local function diff_dirs_diffr(left_dir, right_dir, opt)
   local qf_entries = {}
 
   for _, line in ipairs(lines) do
-    local modified_left, modified_right = line:match('^Files (.+) and (.+) differ$')
+    local modified_left, modified_right = line:match("^Files '?(.-)'? and '?(.-)'? differ$")
     if modified_left and modified_right then
       local left_exists = vim.fn.filereadable(modified_left) == 1
       local right_exists = vim.fn.filereadable(modified_right) == 1

test/functional/plugin/difftool_spec.lua

@@ -15,8 +15,10 @@ setup(function()
   n.mkdir_p(testdir_right)
   t.write_file(testdir_left .. pathsep .. 'file1.txt', 'hello')
   t.write_file(testdir_left .. pathsep .. 'file2.txt', 'foo')
+  t.write_file(testdir_left .. pathsep .. 'file4 with space.txt', 'hello')
   t.write_file(testdir_right .. pathsep .. 'file1.txt', 'hello world') -- modified
   t.write_file(testdir_right .. pathsep .. 'file3.txt', 'bar') -- added
+  t.write_file(testdir_right .. pathsep .. 'file4 with space.txt', 'hello world') -- modified
 end)
 
 teardown(function()
@@ -42,10 +44,12 @@ describe('nvim.difftool', function()
     -- file1.txt as modified (M)
     -- file2.txt as deleted (D)
     -- file3.txt as added (A)
+    -- file4 with space.txt as modified (M)
     eq({
       { text = 'M', rel = 'file1.txt' },
       { text = 'D', rel = 'file2.txt' },
       { text = 'A', rel = 'file3.txt' },
+      { text = 'M', rel = 'file4 with space.txt' },
     }, entries)
   end)
 
@@ -83,6 +87,7 @@ describe('nvim.difftool', function()
     eq({
       { text = 'M', rel = 'file1.txt' },
       { text = 'A', rel = 'file3.txt' },
+      { text = 'M', rel = 'file4 with space.txt' },
     }, entries)
   end)
 
@@ -144,6 +149,7 @@ describe('nvim.difftool', function()
       { text = 'M', rel = 'file1.txt' },
       { text = 'D', rel = 'file2.txt' },
       { text = 'A', rel = 'file3.txt' },
+      { text = 'M', rel = 'file4 with space.txt' },
     }, entries)
   end)
 end)