Make git-send-email detect mbox-style patches more readily

Earlier we insisted that mbox file to begin with "From ".  That
is fine as long as you feed format-patch output, but if you
handcraft the input file, this is unnecessary burden.  We should
detect lines that look like e-mail headers and say that is also
a mbox file.

The other input file format is traditional "send lots of email",
whose first line would never look like e-mail headers, so this
is a safe change.

The original patch was done by Matthew Wilcox, which checked
explicitly for headers the script pays attention to.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-send-email.perl

@@ -481,15 +481,21 @@ foreach my $t (@files) {
 	my $author_not_sender = undef;
 	@cc = @initial_cc;
 	@xh = ();
-	my $found_mbox = 0;
+	my $input_format = undef;
 	my $header_done = 0;
 	$message = "";
 	while(<F>) {
 		if (!$header_done) {
-			$found_mbox = 1, next if (/^From /);
+			if (/^From /) {
+				$input_format = 'mbox';
+				next;
+			}
 			chomp;
+			if (!defined $input_format && /^[-A-Za-z]+:\s/) {
+				$input_format = 'mbox';
+			}
 
-			if ($found_mbox) {
+			if (defined $input_format && $input_format eq 'mbox') {
 				if (/^Subject:\s+(.*)$/) {
 					$subject = $1;
 
@@ -514,6 +520,7 @@ foreach my $t (@files) {
 				# line 1 = cc
 				# line 2 = subject
 				# So let's support that, too.
+				$input_format = 'lots';
 				if (@cc == 0) {
 					printf("(non-mbox) Adding cc: %s from line '%s'\n",
 						$_, $_) unless $quiet;