git-gui: Make Ctrl-T safe to use for conflicting files.

git-gui: Make Ctrl-T safe to use for conflicting files.

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: Alexander Gavrilov <redacted>
Signed-off-by: Shawn O. Pearce <redacted>