etags-regen--update-file: Speed up dramatically

^ turned out to be a very expensive anchor.  It was the difference
between a "real" regexp search and a literal search, and the latter's
much faster.  And since we're searching literally, might as well use
search-forward.
1 job for scratch/etags-regen in 89 minutes and 21 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #10043
test-all

01:29:21

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:966: recipe for target 'check-expensive' failed
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1