(grep-tree-command, grep-tree-files-aliases)

(grep-tree-command, grep-tree-files-aliases)
(grep-tree-ignore-CVS-directories, grep-tree-ignore-case):
New custom variables.
(grep-compute-defaults): Compute grep-tree-command's default value.
(grep-expand-command-macros): New aux function.
(grep-tree-last-regexp, grep-tree-last-files): New aux variables.
(grep-tree): New command like grep-find but extended prompting.