perl/Git.pm: Honor SSH_ASKPASS as fallback if GIT_ASKPASS is not set

If GIT_ASKPASS environment variable is not set, git-svn does not try to use
SSH_ASKPASS as git-core does. This change adds a fallback to SSH_ASKPASS.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

perl/Git.pm

@@ -515,8 +515,8 @@ sub html_path { command_oneline('--html-path') }
 
 Query user C<PROMPT> and return answer from user.
 
-Honours GIT_ASKPASS environment variable for querying
-the user. If no GIT_ASKPASS variable is set or an error occoured,
+Honours GIT_ASKPASS and SSH_ASKPASS environment variables for querying
+the user. If no *_ASKPASS variable is set or an error occoured,
 the terminal is tried as a fallback.
 
 =cut
@@ -527,6 +527,9 @@ sub prompt {
 	if (exists $ENV{'GIT_ASKPASS'}) {
 		$ret = _prompt($ENV{'GIT_ASKPASS'}, $prompt);
 	}
+	if (!defined $ret && exists $ENV{'SSH_ASKPASS'}) {
+		$ret = _prompt($ENV{'SSH_ASKPASS'}, $prompt);
+	}
 	if (!defined $ret) {
 		print STDERR $prompt;
 		STDERR->flush;