send-email: recognize absolute path on Windows

On Windows, absolute paths might start with a DOS drive prefix,
which these two checks failed to recognize.

Unfortunately, we cannot simply use the file_name_is_absolute
helper in File::Spec::Functions, because Git for Windows has an
MSYS-based Perl, where this helper doesn't grok DOS
drive-prefixes.

So let's manually check for these in that case, and fall back to
the File::Spec-helper on other platforms (e.g Win32 with native
Perl)

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1113,6 +1113,18 @@ sub ssl_verify_params {
 	}
 }
 
+sub file_name_is_absolute {
+	my ($path) = @_;
+
+	# msys does not grok DOS drive-prefixes
+	if ($^O eq 'msys') {
+		return ($path =~ m#^/# || $path =~ m#[a-zA-Z]\:#)
+	}
+
+	require File::Spec::Functions;
+	return File::Spec::Functions::file_name_is_absolute($path);
+}
+
 # Returns 1 if the message was sent, and 0 otherwise.
 # In actuality, the whole program dies when there
 # is an error sending a message.
@@ -1197,7 +1209,7 @@ sub send_message {
 
 	if ($dry_run) {
 		# We don't want to send the email.
-	} elsif ($smtp_server =~ m#^/#) {
+	} elsif (file_name_is_absolute($smtp_server)) {
 		my $pid = open my $sm, '|-';
 		defined $pid or die $!;
 		if (!$pid) {
@@ -1271,7 +1283,7 @@ sub send_message {
 		printf (($dry_run ? "Dry-" : "")."Sent %s\n", $subject);
 	} else {
 		print (($dry_run ? "Dry-" : "")."OK. Log says:\n");
-		if ($smtp_server !~ m#^/#) {
+		if (!file_name_is_absolute($smtp_server)) {
 			print "Server: $smtp_server\n";
 			print "MAIL FROM:<$raw_from>\n";
 			foreach my $entry (@recipients) {