Simplify terminal character style diff

[tlinford]

These changes make the code a bit simpler, and avoid outputting ansi reset escape codes each time an EMPTY_TERMINAL_CHARACTER is found.

Example output to terminal emulator:

• before:

"\x1b[3;2H\x1b[mWelcome to fish, the friendly interactive shell\x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m \x1b[m "

• after:

"\x1b[3;2H\x1b[mWelcome to fish, the friendly interactive shell\x1b[m                                                                 "