We would like our default pager of `less -FRX` to also get passed
`LESSCHARSET=utf-8`. I don't like having defaults that the user can't
specify themselves, so this commits provides users with a way to pass
environment variables to their configured pager (or editor, or merge
tool, ...).
I didn't document this feature and I didn't add it to the config
schema because it seems it's going to be so rarely useful to users.
The assumption here is temp_dir wouldn't contain invalid utf-8 bytes. If it
can contain invalid bytes, maybe we can remove temp_dir from arguments, and
chdir(temp_dir) instead.
This unblocks the use of Regex. We could use regex::bytes, but it's way
more complex as we would have to go back and forth between str/OsStr and
bytes.
The default edit_args will be changed to ["$left", "$right"] to support
variable substitution without breaking the existing configuration too much.
The default merge_args could also be set if we could come up with something
meaningful.
If FullCommandArgs is renamed to CommandNameAndArgs, the meaning of .args()
will get fuzzy. This also clarifies that the name part exists even if the
source command string is empty.
For stock merge-tools, having name -> args indirection makes sense. For
user-specific settings, it's simpler to set command name and arguments
together.
It might be a bit odd that "name with whitespace" can be parsed differently
depending on the existence of merge-tools."name with whitespace".
editor_name_from_settings() needs &mut Ui to show hint, but we're lucky that
the caller has a clone of UserSettings. This is one reason I want to remove
ui.settings(). A function taking (&mut Ui, &UserSettings) sounds reasonable,
but we can't pass in (&mut ui, ui.settings()) to it.
