Fix issue with fmt: skip (#2254)

Not sure the fix is right.  Here is what I found: issue is connected
with line

    first.prefix = prefix[comment.consumed :]

in `comments.py`.  `first.prefix` is a prefix of the line, that ends
with `# fmt: skip`, but `comment.consumed` is the length of the
`"  # fmt: skip"` string.  If prefix length is greater than 14,
`first.prefix` will grow every time we apply formatting.

src/black/comments.py

@@ -159,7 +159,10 @@ def convert_one_fmt_off_pair(node: Node) -> bool:
             first = ignored_nodes[0]  # Can be a container node with the `leaf`.
             parent = first.parent
             prefix = first.prefix
-            first.prefix = prefix[comment.consumed :]
+            if comment.value in FMT_OFF:
+                first.prefix = prefix[comment.consumed :]
+            if comment.value in FMT_SKIP:
+                first.prefix = ""
             hidden_value = "".join(str(n) for n in ignored_nodes)
             if comment.value in FMT_OFF:
                 hidden_value = comment.value + "\n" + hidden_value

tests/data/fmtskip6.py

@@ -0,0 +1,5 @@
+class A:
+    def f(self):
+        for line in range(10):
+            if True:
+                pass  # fmt: skip