Don't remove quotes if \ or " are present inside

This is for single line quoting in the ConfigParser.

This leaves the changes in #2035 (as adjusted in #2036) intact for
the cases where it addressed #1923: when the `...` in `"..."`
(appearing in the value position on a single `{name} = {value}"`
line) has no occurrences of `\` or `"`, quote removal is enough.

But when `\` or `"` does appear, this suppresses quote removal.
This is with the idea that, while it would be better to interpret
such lines as Git does, we do not yet do that, so it is preferable
to return the same results we have in the past (which some programs
may already be handling themselves).

This should make the test introduced in the preceding commit pass.
But it will be even better to support more syntax, at least
well-formed escapes. As noted in the test, both the test and the
code under test can be adjusted for that.

(See comments in #2035 for context.)

git/config.py

@@ -505,13 +505,16 @@ def string_decode(v: str) -> str:
                     optval = optval.strip()
 
                     if len(optval) < 2 or optval[0] != '"':
-                        pass  # Nothing to treat as opening quotation.
+                        # Does not open quoting.
+                        pass
                     elif optval[-1] != '"':
+                        # Opens quoting and does not close: appears to start multi-line quoting.
                         is_multi_line = True
                         optval = string_decode(optval[1:])
-                    # END handle multi-line
-                    else:
+                    elif optval.find("\\", 1, -1) == -1 and optval.find('"', 1, -1) == -1:
+                        # Opens and closes quoting. Single line, and all we need is quote removal.
                         optval = optval[1:-1]
+                    # TODO: Handle other quoted content, especially well-formed backslash escapes.
 
                     # Preserves multiple values for duplicate optnames.
                     cursect.add(optname, optval)