Export three helper functions from ls-files

Export three helper functions from ls-files

This exports three helper functions from ls-files.

 * pathspec_match() checks if a given path matches a set of pathspecs
   and optionally records which pathspec was used.  This function used
   to be called "match()" but renamed to be a bit less vague.

 * report_path_error() takes a set of pathspecs and the record
   pathspec_match() above leaves, and gives error message.  This
   was split out of the main function of ls-files.

 * overlay_tree_on_cache() takes a tree-ish (typically "HEAD")
   and overlays it on the current in-core index.  By iterating
   over the resulting index, the caller can find out the paths
   in either the index or the HEAD.  This function used to be
   called "overlay_tree()" but renamed to be a bit more
   descriptive.

Signed-off-by: Junio C Hamano <gitster@pobox.com>