git-svn: Support retrieving passwords with GIT_ASKPASS

git-svn reads passwords from an interactive terminal.
This behavior cause GUIs to hang waiting for git-svn to
complete

Fix this problem by allowing a password-retrieving command
to be specified in GIT_ASKPASS. SSH_ASKPASS is supported
as a fallback when GIT_ASKPASS is not provided.

Signed-off-by: Frank Li <lznuaa@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-svn.perl

@@ -3970,18 +3970,25 @@ sub username {
 
 sub _read_password {
 	my ($prompt, $realm) = @_;
-	print STDERR $prompt;
-	STDERR->flush;
-	require Term::ReadKey;
-	Term::ReadKey::ReadMode('noecho');
 	my $password = '';
-	while (defined(my $key = Term::ReadKey::ReadKey(0))) {
-		last if $key =~ /[\012\015]/; # \n\r
-		$password .= $key;
+	if (exists $ENV{GIT_ASKPASS}) {
+		open(PH, "-|", $ENV{GIT_ASKPASS}, $prompt);
+		$password = <PH>;
+		$password =~ s/[\012\015]//; # \n\r
+		close(PH);
+	} else {
+		print STDERR $prompt;
+		STDERR->flush;
+		require Term::ReadKey;
+		Term::ReadKey::ReadMode('noecho');
+		while (defined(my $key = Term::ReadKey::ReadKey(0))) {
+			last if $key =~ /[\012\015]/; # \n\r
+			$password .= $key;
+		}
+		Term::ReadKey::ReadMode('restore');
+		print STDERR "\n";
+		STDERR->flush;
 	}
-	Term::ReadKey::ReadMode('restore');
-	print STDERR "\n";
-	STDERR->flush;
 	$password;
 }