Allow the user to change the temporary file name for mergetool

Using the original filename suffix for the temporary input files to
the merge tool confuses IDEs like Eclipse. This patch introduces
a configurtion option, mergetool.tmpsuffix, which get appended to
the temporary file name. That way the user can choose to use a
suffix like ".tmp", which does not cause confusion.

Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>