gitweb: correctly store previous rev in javascript-actions mode

Without this change, the setting

 $feature{'javascript-actions'}{'default'} = [1];

in gitweb.conf breaks gitweb's blame page: clicking on line numbers
displayed in the second column on the page has no effect.

For comparison, with javascript-actions disabled, clicking on line
numbers loads the previous version of the line.

Addresses https://bugs.debian.org/741883.

Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Robert Luberda <redacted>
Acked-by: Jakub Narębski <redacted>
Signed-off-by: Junio C Hamano <redacted>