send-email: use catfile() to concatenate files

Change send-email to use Perl's catfile() function instead of
"$dir/$file". If send-email is given a $dir that ends with a / we'll
end up printing a double slashed path like "dir//mtfnpy.patch".

This doesn't cause any problems since Perl's IO layer will handle it,
but it looks ugly.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -24,6 +24,7 @@
 use Data::Dumper;
 use Term::ANSIColor;
 use File::Temp qw/ tempdir tempfile /;
+use File::Spec::Functions qw(catfile);
 use Error qw(:try);
 use Git;
 
@@ -511,7 +512,7 @@ ($)
 		opendir(DH,$f)
 			or die "Failed to opendir $f: $!";
 
-		push @files, grep { -f $_ } map { +$f . "/" . $_ }
+		push @files, grep { -f $_ } map { catfile($f, $_) }
 				sort readdir(DH);
 		closedir(DH);
 	} elsif ((-f $f or -p $f) and !check_file_rev_conflict($f)) {