A previous patch added a check for conflict markers, which
is done when the file is about to be staged due to a click
on the icon. However, pressing Ctrl-T still immediately
stages the file without confirmation. This patch fixes it.

The check requires a loaded diff, so staging multiple files
at once won't work if they are unmerged.
Signed-off-by: NAlexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>