matching before recursing instead of after.  The DFA match eliminates
unworkable midpoint choices a lot faster than the recursive check, in most
cases, so doing it first can speed things up; particularly in pathological
cases such as recently exhibited by Michael Glaesemann.

In addition, apply some cosmetic changes that were applied upstream (in the
Tcl project) at the same time, in order to sync with upstream version 1.15
of regexec.c.

Upstream apparently intends to backpatch this, so I will too.  The
pathological behavior could be unpleasant if encountered in the field,
which seems to justify any risk of introducing new bugs.

Tom Lane, reviewed by Donal K. Fellows of Tcl project