send-email: implement sendmail aliases line continuation support

Logical lines in sendmail aliases files can be spread over multiple
physical lines[1]. A line beginning with whitespace is folded into the
preceding line. A line ending with '\' consumes the following line.

[1]: https://www.freebsd.org/cgi/man.cgi?query=aliases&sektion=5

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -394,8 +394,6 @@ described below:
 sendmail;;
 *	Quoted aliases and quoted addresses are not supported: lines that
 	contain a `"` symbol are ignored.
-*	Line continuations are not supported: lines that start with
-	whitespace characters, or end with a `\` symbol are ignored.
 *	Redirection to a file (`/path/name`) or pipe (`|command`) is not
 	supported.
 *	File inclusion (`:include: /path/name`) is not supported.

git-send-email.perl

@@ -492,8 +492,6 @@ sub parse_sendmail_alias {
 	local $_ = shift;
 	if (/"/) {
 		print STDERR "warning: sendmail alias with quotes is not supported: $_\n";
-	} elsif (/^\s|\\$/) {
-		print STDERR "warning: sendmail continuation line is not supported: $_\n";
 	} elsif (/^(\S+?)\s*:\s*(.+)$/) {
 		my ($alias, $addr) = ($1, $2);
 		$aliases{$alias} = [ split_addrs($addr) ];
@@ -504,10 +502,16 @@ sub parse_sendmail_alias {
 
 sub parse_sendmail_aliases {
 	my $fh = shift;
+	my $s = '';
 	while (<$fh>) {
+		chomp;
 		next if /^\s*$/ || /^\s*#/;
-		parse_sendmail_alias($_);
+		$s .= $_, next if $s =~ s/\\$// || s/^\s+//;
+		parse_sendmail_alias($s) if $s;
+		$s = $_;
 	}
+	$s =~ s/\\$//; # silently tolerate stray '\' on last line
+	parse_sendmail_alias($s) if $s;
 }
 
 my %parse_alias = (