FR: Consider detecting changes by looking at size+timestamp only

[krlmlr]

Use case: Large SQLite database which I'd rather not have read entirely just to detect it hasn't changed.

Should be configurable per target and as a global option, or even making it the default. (How often does a file keep its size and mod time when contents change?)

Also, a file should be considered unchanged (with its cached timestamp information updated) if the hash is unchanged (means size unchanged but timestamp different). This could also be implemented in a different option.