git-gui: Don't allow staging files with conflicts.

git-gui: Don't allow staging files with conflicts.

Prevent staging files with conflict markers by clicking
on the icon in the 'Unstaged Changes' list. Instead, pretend
that the user clicked the name, and show the diff.

Originally it made some sense to allow staging conflicting
files, because git-gui did not provide any tools to resolve
them from within the GUI. But now that we have added mergetool
capabilities, it is more likely to cause accidental and
non-undoable errors.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>