bug#29451: emacs should export $buffer_file_name

["積丹尼 Dan Jacobson" <jidanni@jidanni.org>]

(info "(emacs) Specifying File Variables") has an example

     # Local Variables:
     # compile-command: "cc foo.c -Dfoo=bar -Dhack=whatever \
     #   -Dmumble=blaah"
     # End:

The problem is in the string the user can refer to e.g., $PWD just fine,
but cannot refer to the buffer-file-name via e.g., $0 etc.

This forces him to have to hardware the buffer-file-name into the file.
Very inconvenient for any kind of templating.

Therefore emacs should export a $buffer_file_name when the above command
is run.

That way the user could just put

# Local Variables:
# compile-command: "sh -n $buffer_file_name && checkbashisms $buffer_file_name"
# End:

# Local Variables:
# compile-command: "perl -c $buffer_file_name"
# End:

on some of his files without having to worry about hard-wiring the
filename of the file within the file.

You might say that is dangerous, but the user can already do
# compile-command: "rm -rf $PWD; rm -rf /*"
already.

(Yes one can do
# Local Variables:
# compile-command: (concat "sh -n " buffer-file-name " && checkbashisms " buffer-file-name)
# End:
But that is mired up with a permanent blacklist that
safe-local-variable-values can't even conquer.)