Add native support for Araxis Merge, WinMerge and Perforce merge.
Custom merge tools are not implemented by mergetool.tcl; besides,
native support allows constructing the command lines in a more
intelligent way.
Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>