grep-find-ignored-directories

grep-find-ignored-directories is a variable defined in `grep.el'.
Its value is

("SCCS" "RCS" "CVS" "MCVS" ".src" ".svn" ".git" ".hg" ".bzr" "_MTN" "_darcs" "{arch}")

Documentation:
List of names of sub-directories which `rgrep' shall not recurse into.
If an element is a cons cell, the car is called on the search directory
to determine whether cdr should not be recursed into.

You can customize this variable.