Encode non-ascii headers (e.g. accented characters in Subject line)
Rename HEADER_TEMPLATE to REFCHANGE_HEADER_TEMPLATE.
Split up HEADER_TEMPLATES.

Split *_HEADER_TEMPLATE into *_HEADER_TEMPLATE and *_INTRO_TEMPLATE,
where the former contains only the email header lines and the latter
contains the text forming the beginning of the email body.  This (1)
will make it easier to handle the email headers in a more RFC
2822-compliant manner, and (2) is a better division for users who
might want to customize one or the other.
Split off method Change.generate_email_intro().

Analogous to what was just done for the HEADER_TEMPLATES, split up
Change.generate_email_header() into generate_email_header() and
generate_email_intro().
Expand header lines and body lines differently.

Only silently skip lines containing unknown variables references when
expanding header lines.  When expanding body lines, make it an error
if there to be an unknown variable reference.
Merge branch 'non-ascii-header' into header-handling

If a header contains non-ASCII characters, assume that it is UTF-8 and
encode it appropriately for an email header.

Conflicts:
	git-multimail/git_multimail.py
Properly handle header lines that are too long.

If a header line is too long, email.header.Header.encode() splits it
into multiple lines with proper line continuation characters.  Since
later code expects to receive lines one at a time, such lines need to
be re-split and yielded one by one.  The easiest way to do that is to
move the encoding functionality into Change.expand_header_lines(),
where (1) we can split the lines into name and value *before*
expansion and (2) we can emit as many lines as we want.  Also tell the
Header instance the name of the header so that it can account for that
length when splitting the first line.
Merge branch 'header-handling'

Handle email headers separately from the email body and treat them in
a more RFC 2822-conforming way:

* Split the HEADER_TEMPLATEs into *_HEADER_TEMPLATE and
  *_INTRO_TEMPLATE

* Extract a method Change.generate_email_intro().

* Only silently ignore unknown expansion values in header lines (not
  in the body).

* Encode non-ASCII characters that appear in the email headers.

* Split header lines that are too long.