git-gui: Reenable staging unmerged files by clicking the icon.

git-gui: Reenable staging unmerged files by clicking the icon.

This restores functionality of the file icon for unmerged files.
Safety is enforced by loading the diff and checking for lines
that look like conflict markers. If such lines are found, or
the conflict involves deletion and/or symlinks, a confirmation
dialog is presented. Otherwise, the icon immediately stages the
working copy version of the file.

Includes a revert of 2fe5b2ee42897a3acc78e5ddaace3775eb2713ca
(Restore ability to Stage Working Copy for conflicts)

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Tested-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>