mergetool: use more conservative temporary filenames

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 <redacted>
Helped-by: Junio C Hamano <redacted>
Signed-off-by: David Aguilar <redacted>
Signed-off-by: Junio C Hamano <redacted>