Refactor quote parsing, to prepare for more checks

This refactors ConfigParser double-quote parsing near the single
line double-quoted value parsing code, so that:

- Code that parses the name is less intermixed with code that
  parses the value.

- Conditional logic is less duplicated.

- The `END` comment notation appears next to the code it describes.

- The final `else` can be turned into one or more `elif` followed
  by `else` to cover different cases of `"..."` differently. (But
  those are not added here. This commit is purely a refactoring.)

(The `pass` suite when `len(optval) < 2 or optval[0] != '"'` is
awkward and not really justified right now, but it looks like it
may be able to help with readabilty and help keep nesting down
when new `elif` cases are added.)

git/config.py

@@ -496,18 +496,23 @@ def string_decode(v: str) -> str:
                 if mo:
                     # We might just have handled the last line, which could contain a quotation we want to remove.
                     optname, vi, optval = mo.group("option", "vi", "value")
+                    optname = self.optionxform(optname.rstrip())
+
                     if vi in ("=", ":") and ";" in optval and not optval.strip().startswith('"'):
                         pos = optval.find(";")
                         if pos != -1 and optval[pos - 1].isspace():
                             optval = optval[:pos]
                     optval = optval.strip()
-                    optname = self.optionxform(optname.rstrip())
-                    if len(optval) > 1 and optval[0] == '"' and optval[-1] != '"':
+
+                    if len(optval) < 2 or optval[0] != '"':
+                        pass  # Nothing to treat as opening quotation.
+                    elif optval[-1] != '"':
                         is_multi_line = True
                         optval = string_decode(optval[1:])
-                    elif len(optval) > 1 and optval[0] == '"' and optval[-1] == '"':
-                        optval = optval[1:-1]
                     # END handle multi-line
+                    else:
+                        optval = optval[1:-1]
+
                     # Preserves multiple values for duplicate optnames.
                     cursect.add(optname, optval)
                 else: