[PATCH v3] Re: git-am: fix maildir support regression: accept email file as patch

>     The point of this is not to use the silly "we only look at the first
>     three lines" rule.  Instead, it ignores these l1/l2/l3, but grabs all
>     the header lines, but discards second and subsequent physical lines if
>     a logical line was folded.  Which means that the effect of this is to
>     pipe the whole header (again, without worrying about the indented
>     remainder of folded lines) to downsream, which is the grep -v below
> 
>         grep -v -E -e '^[A-Za-z]+(-[A-Za-z]+)*:' >/dev/null ||
>
>     This checks if there is a line that does _NOT_ match the usual
>     e-mail header pattern.  If there is such a line, it means that the
>     file is not an e-mail message.  If there is no such line, we say...

> One caveat is that the above logic alone won't catch a random file that
> does not have _any_ e-mail headers in it.  So you might need to do
> something like:
> 
> 	LF='
>         '
> 	case "$l1$LF$l2$LF$l3" in
>         *"$LF$LF"*)
>         	# has a completely empty line in there?
>                 # that means the message has only two headers at most;
>                 # that cannot be an email.
> 		;;