Don't omit escaping the second consecutive quote

This would produce invalid code for strings like `"x = ''; y = \"\""`.

black.py

@@ -1998,6 +1998,9 @@ def normalize_string_quotes(leaf: Leaf) -> None:
     else:
         new_body = escaped_orig_quote.sub(rf"\1{orig_quote}", body)
         new_body = unescaped_new_quote.sub(rf"\1\\{new_quote}", new_body)
+        # Add escapes again for consecutive occurences of new_quote (sub
+        # doesn't match overlapping substrings).
+        new_body = unescaped_new_quote.sub(rf"\1\\{new_quote}", new_body)
     if new_quote == '"""' and new_body[-1] == '"':
         # edge case:
         new_body = new_body[:-1] + '\\"'

tests/string_quotes.py

@@ -30,6 +30,14 @@
 the \'lazy\' dog.\n\
 '
 re.compile(r'[\\"]')
+"x = ''; y = \"\""
+"x = '''; y = \"\""
+"x = ''''; y = \"\""
+"x = '' ''; y = \"\""
+"x = ''; y = \"\"\""
+"x = '''; y = \"\"\"\""
+"x = ''''; y = \"\"\"\"\""
+"x = '' ''; y = \"\"\"\"\""
 
 # output
 
@@ -65,3 +73,11 @@
 the 'lazy' dog.\n\
 "
 re.compile(r'[\\"]')
+"x = ''; y = \"\""
+"x = '''; y = \"\""
+"x = ''''; y = \"\""
+"x = '' ''; y = \"\""
+'x = \'\'; y = """'
+'x = \'\'\'; y = """"'
+'x = \'\'\'\'; y = """""'
+'x = \'\' \'\'; y = """""'