Open external merge tool with original file extensions for all three files

Open external merge tool with original file extensions for all three files

Before this change, conflicted files were open in external merge tool with
temporary filenames like REMOTE.$$ and LOCAL.$$.  This way meld was unable
to recognize these files and syntax highlighting feature was unusable.

Help such merge tools by giving temporar files the same extension as the
original.

Signed-off-by: Pini Reznik <pinir@expand.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>