bug#61514: 30.0.50; sadistically long xml line hangs emacs

[Gregory Heytings]

> SGTM, but isn't 1000 a somewhat low value?  What if we use half of the 
> value of long-line-optimizations-region-size instead?

Here are some benchmarks.  The time taken by Emacs to open the 4 MB 
"n_n_..." file with different regexps are:

"[^<>\n]\\{1,100\\}?\\<": 0.8 seconds
"[^<>\n]\\{1,1000\\}?\\<": 3.4 seconds
"[^<>\n]\\{1,10000\\}?\\<": 28.5 seconds
"[^<>\n]\\{1,65535\\}?\\<": 162.9 seconds
"[^<>\n]+?\\<": 356.6 seconds

65535 is the upper limit for such ranges, it's not possible to use a 
larger value.