mergetool: add an option for writing to a temporary directory

mergetool: add an option for writing to a temporary directory

Teach mergetool to write files in a temporary directory when
'mergetool.writeToTemp' is true.

This is helpful for tools such as Eclipse which cannot cope with
multiple copies of the same file in the worktree.

Suggested-by: Charles Bailey <redacted>
Signed-off-by: David Aguilar <redacted>
Signed-off-by: Junio C Hamano <redacted>