git-gui: Support calling merge tools.

git-gui: Support calling merge tools.

Adds an item to the diff context menu in conflict mode,
which invokes a merge tool for the selected file. Tool
command-line handling code was ported from git-mergetool.

Automatic default tool selection and custom merge tools
are not supported. If merge.tool is not set, git-gui
defaults to meld.

This implementation uses a checkout-index hack in order
to retrieve all stages with autocrlf and filters properly
applied. It requires temporarily moving the original
conflict file out of the way.

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