emacs: `with-current-notmuch-show-message' should not leak `coding-system-for-read'

`with-current-notmuch-show-message' applies a `no-conversion' coding
system when reading a raw message from notmuch. That coding system
should _not_ be applied when the body of the macro is evaluated, as it
can cause file operations used during that evaluation to incorrectly
apply the `no-conversion' coding system.

This was discovered when a user's .signature file contained non-ASCII
characters. When a message is forwarded, the `no-conversion' coding
system was applied to the reading of the .signature file, resulting in
raw rather than UTF-8 interpretation of the data.