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>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>