Handle non-ASCII paths.

This prevents git-ls-files from escaping "unusual" characters in
pathnames.  When this happened, gitgutter would feed the escaped name
back to git-show but git-show would not recognise it.

This commit makes git-ls-files use the -z option to output pathnames
verbatim.  These pathnames also become null terminated so we have to
ensure we remove the null terminator too.

Closes #562.

test/test_gitgutter.vim

@@ -191,6 +191,20 @@ function Test_filename_leading_dash()
 endfunction
 
 
+function Test_filename_umlaut()
+  call system('touch -- fixtüre.txt && git add -- fixtüre.txt')
+  edit fixtüre.txt
+  normal ggo*
+  call s:trigger_gitgutter()
+
+  let expected = [
+        \ 'line=1  id=3000  name=GitGutterLineAdded',
+        \ 'line=2  id=3001  name=GitGutterLineAdded'
+        \ ]
+  call assert_equal(expected, s:signs('fixtüre.txt'))
+endfunction
+
+
 " FIXME: this test fails when it is the first (or only) test to be run
 function Test_follow_symlink()
   let tmp = 'symlink'