dim: don't blow up on newline in Message-Id

Patchwork adds trailing newlines to long Message-Id header fields.
message_get_id returns a string containing the newline, and
dim_commit_add_tag blows up on that:

    sed: -e expression #1, char 57: unknown command: `<'

Signed-off-by: Simon Ser <contact@emersion.fr>
Closes: #17


Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>