Test a quoted config var with meaningful edge whitespace

#2035 fixed issue #1923 by removing separate double quotation marks
appearing on a single-line configuration variable when parsing a
configuration file. However, it also stripped leading and trailing
whitespace from the string obtained by removing the quotes.

This adds a test case of a plausible scenario where such whitespace
needs to be preserved and where a user would almost certainly expect
it to preserve: setting a value like `# ` for `core.commentString`,
in order to be able to easily create commit messages like this one,
that contain a line that begins with a literal `#`, while still
letting `#` in the more common case that it is followed by a space
be interpreted as a comment.

The effect of  `git config --local core.commentString '# '` is to
add a `commentString = "# "` line in the `[core]` section of
`.git/config`. The changes in #2035 allow us to correctly parse
more quoted strings than before, and almost allow us to parse this,
but not quite, because of the `strip()` operation that turns `# `
into `#`.

Author: EliahKagan

test/fixtures/git_config_with_quotes_whitespace

@@ -0,0 +1,2 @@
+[core]
+  commentString = "# "

test/test_config.py

@@ -412,6 +412,10 @@ def test_config_with_quotes(self):
         self.assertEqual(cr.get("user", "name"), "Cody Veal")
         self.assertEqual(cr.get("user", "email"), "cveal05@gmail.com")
 
+    def test_config_with_quotes_with_literal_whitespace(self):
+        cr = GitConfigParser(fixture_path("git_config_with_quotes_whitespace"), read_only=True)
+        self.assertEqual(cr.get("core", "commentString"), "# ")
+
     def test_get_values_works_without_requiring_any_other_calls_first(self):
         file_obj = self._to_memcache(fixture_path("git_config_multiple"))
         cr = GitConfigParser(file_obj, read_only=True)