mergetool: use more conservative temporary filenames

Avoid filenames with multiple dots so that overly-picky tools do
not misinterpret their extension.

Previously, foo/bar.ext in the worktree would result in e.g.

	foo/bar.ext.BASE.1234.ext

This can be improved by having only a single .ext and using
underscore instead of dot so that the extension cannot be
misinterpreted.  The resulting path becomes:

	foo/bar_BASE_1234.ext

Suggested-by: Sergio Ferrero <sferrero@ensoftcorp.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'da/mergetool-temporary-filename' into jch

* da/mergetool-temporary-filename:
  mergetool: use more conservative temporary filenames