Merge files in directories DIR1 and DIR2 using files in ANCESTOR-DIR as ancestors.
Ediff merges files that have identical names in DIR1, DIR2. If a pair of files
in DIR1 and DIR2 doesn't have an ancestor in ANCESTOR-DIR, Ediff will merge
without ancestor. The fourth argument, REGEXP, is nil or a regular expression;
only file names that match the regexp are considered.
(fn DIR1 DIR2 ANCESTOR-DIR REGEXP &optional MERGE-AUTOSTORE-DIR)