Add dummy 'shortmess' option, which does nothing
Fix a typo in engine/options.h
Add OPT_CHARSET option type to appropriate enum
Prepare tests skeleton for new option type tests
Assert in set_set on unknown type of option
Implement support of OPT_CHARSET type of options

It supports the following type of operations: =, +=, -=.
Add missing update of option change flag
Use OPT_CHARSET for 'cpoptions' and 'shortmess'
Fail on unknown characters for = and += operations
Provide proper completion for charset options
Simplify one of statements in complete_options()
Document charset type of options
Add missing update of option change flag

Yes, another one.
Return zero on memory fail in replace_if_changed()

Was ignored before.
Use replace_if_changed() in one more place
Explicitly specify type of terminal for tests

This is needed to optimize execution time of some of them, which
traverse color pairs (64 - for 8 color terminal, but 32768!! - for 256
color terminal).
Add tests for change notification on option reset

Specifically to string option.
Extrace notify_option_update() function in options
Add "T" flag to the 'shortmess' option

For not it just sets value of the cfg.trunc_normal_sb_msgs field.
Use replace_string() in status_bar_message_i()