The thread at http://thread.gmane.org/gmane.comp.version-control.git/257392
details problems when applying patches with "git am" in a repository with
CRLF line endings.  In the example in the thread, the repository originated
from "git-svn" so it is not possible to use core.eol and friends on it.

Right now, the best option is to use "git am --keep-cr".  However, when
a patch create new files, the patch application process will reject the
new file because it finds a "/dev/null\r" string instead of "/dev/null".

The problem is that SMTP transport is CRLF-unsafe.  Sending a patch by
email is the same as passing it through "dos2unix | unix2dos".  The newly
introduced CRLFs are normally transparent because git-am strips them. The
keepcr=true setting preserves them, but it is mostly working by chance
and it would be very problematic to have a "git am" workflow in a
repository with mixed LF and CRLF line endings.

The MIME solution to this is the quoted-printable transfer enconding.
This is not something that we want to enable by default, since it makes
received emails horrible to look at.  However, it is a very good match
for projects that store CRLF line endings in the repository.

The only disadvantage of quoted-printable is that quoted-printable
patches fail to apply if the maintainer uses "git am --keep-cr".  This
is because the decoded patch will have two carriage returns at the end
of the line.  Therefore, add support for base64 transfer encoding too,
which makes received emails downright impossible to look at outside
a MUA, but really just works.

The patch covers all bases, including users that still live in the late
80s, by also providing a 7bit content transfer encoding that refuses
to send emails with non-ASCII character in them.  And finally, "8bit"
will add a Content-Transfer-Encoding header but otherwise do nothing.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>