add--interactive: fix missing file prompt for patch mode with "-i"

When invoked as "git add -i", each menu interactive menu option prompts the user to select a list of files. This includes the "patch" option, which gets the list before starting the hunk-selection loop. As "git add -p", it behaves differently, and jumps straight to the hunk selection loop. Since 0539d5e6d (i18n: add--interactive: mark patch prompt for translation, 2016-12-14), the "add -i" case mistakenly jumps to straight to the hunk-selection loop. Prior to that commit the distinction between the two cases was managed by the $patch_mode variable. That commit used $patch_mode for something else, and moved the old meaning to the "$cmd" variable. But it forgot to update the $patch_mode check inside patch_update_cmd() which controls the file-list behavior. The simplest fix would be to change that line to check $cmd. But while we're here, let's use a less obscure name for this flag: $patch_mode_only, a boolean which tells whether we are in full-interactive mode or only in patch-mode. Reported-by: Henrik Grubbström <grubba@grubba.org> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>