Fix fish_prompt for special working directories

[arkbriar]

Before the change, working under directories like /t/mm will print a prompt like /t/m m >>> where you can find an unexpected blank in the middle. I didn't look into the exact cause but I presume it was probably caused by the escaped ANSI color commands (where m is a common character). The PR also fixes a problem for the substitution of ~ for those directories like /t/a~.

The fix applies to the sed command to

• Restrict the substitution to the ~ at the beginning, which is supposed to be the home.
• Restrict the substitution to the $base before the end, which is the only right location.
• Add the trailing blank for home. Before the change it's ^^>>>, now it should be ^^ >>> correctly.