Unstable formatting on split string + % formatting

[JelleZijlstra]

This is on master.

$ cat kill_senior_procs.py 
if __name__ == "__main__":
    for i in range(4, 8):
        cmd = (
            r"for pid in $(ps aux | grep paster | grep -v grep | grep '\-%d' | awk '{print $2}'); do kill $pid; done"
            % (i)
        )
$ black kill_senior_procs.py 
error: cannot format kill_senior_procs.py: INTERNAL ERROR: Black produced different code on the second pass of the formatter.  Please report a bug on https://github.com/psf/black/issues.  This diff might be helpful: /var/folders/wx/zkst6jdx1zg_s32d23gz4_0r0000gn/T/blk_bgpw4vyu.log
Oh no! 💥 💔 💥
1 file failed to reformat.
$ cat /var/folders/wx/zkst6jdx1zg_s32d23gz4_0r0000gn/T/blk_bgpw4vyu.log
--- source
+++ first pass
@@ -1,6 +1,7 @@
 if __name__ == "__main__":
     for i in range(4, 8):
         cmd = (
-            r"for pid in $(ps aux | grep paster | grep -v grep | grep '\-%d' | awk '{print $2}'); do kill $pid; done"
+            r"for pid in $(ps aux | grep paster | grep -v grep | grep '\-%d' | awk"
+            r" '{print $2}'); do kill $pid; done"
             % (i)
         )
--- first pass
+++ second pass
@@ -1,7 +1,6 @@
 if __name__ == "__main__":
     for i in range(4, 8):
         cmd = (
             r"for pid in $(ps aux | grep paster | grep -v grep | grep '\-%d' | awk"
-            r" '{print $2}'); do kill $pid; done"
-            % (i)
+            r" '{print $2}'); do kill $pid; done" % (i)
         )

[hugovk]

And a fourth git bisect pointing to 544ea9c from "Improve String Handling" (#1132).

[hugovk]

Also now good on master

$ black --check kill_senior_procs.py
All done! ✨ 🍰 ✨
1 file would be left unchanged.