Improve compression ratio of the --patch-from mode

[Cyan4973]

--patch-from mode essentially combines --long with dictionary, in order to create a very small patch when compressing a new document based on an older revision of the same document.

The --long mode features advanced parameters, that can be tuned by the user, but are rarely employed. As a consequence, the "default" values are almost always used, resulting in non-optimal performance.

This patch dynamically updates the advanced parameters of the --long mode, spending more cpu time to favor compression ratio at higher levels.

For illustration, here is an extract of the updated performance, using the following scenario:
zstd -# -T6 ~/dev/bench/linux-v6.13.tar --patch-from ~/dev/bench/linux-v6.12.tar
running on a ubuntu desktop with a i7-9700k cpu:

Level	strategy	dev time	PR time	dev cSize	PR cSize	ratio improv.
1	fast	2.81s	2.99s	7,806,350	7,619,530	2.45%
3	dfast	2.83s	3.01s	7,526,673	7,347,628	2.44%
5	greedy	2.85s	3.51s	7,992,373	7,429,686	7.57%
6	lazy	2.86s	3.52s	7,719,580	7,233,837	6.71%
8	lazy2	2.89s	3.69s	7,450,059	6,960,293	7.04%
13	btlazy2	9.2s	10s	6,829,065	6,048,655	12.90%
16	btopt	30s	31s	6,076,109	5,421,444	12.08%
18	btultra	41s	42s	5,780,344	5,075,507	13.89%
19	btultra2	59s	62s	5,522,028	4,724,116	16.89%

[terrelln]

nit: Is this comment out of date?

[Cyan4973]

Ah yes, indeed,
it should be from 7 to 4 now

to be updated