This implements support for numeric flags without an associated short or
long flag name. This pattern is used by many commands. For example `head
-3 /a/file` to emit the first three lines of the file.
Fixes#4214
We need a way to encode arbitrary strings into valid fish variable
names. It would also be nice if we could convert strings to valid URLs
without using the slow and hard to understand `__fish_urlencode` function.
In particular, eliminating the need to manipulate the locale.
Fixes#4150
This implements `status is-breakpoint` that returns true if the current
shell prompt is displayed in the context of a `breakpoint` command.
This also fixes several bugs. Most notably making `breakpoint` a no-op if
the shell isn't interactive. Also, typing `breakpoint` at an interactive
prompt should be an error rather than creating a new nested debugging
context.
Partial fix for #1310
