builtin-grep: make pieces of it available as library.

builtin-grep: make pieces of it available as library.

This makes three functions and associated option structures from
builtin-grep available from other parts of the system.

 * options to drive built-in grep engine is stored in struct
   grep_opt;

 * pattern strings and extended grep expressions are added to
   struct grep_opt with append_grep_pattern();

 * when finished calling append_grep_pattern(), call
   compile_grep_patterns() to prepare for execution;

 * call grep_buffer() to find matches in the in-core buffer.

This also adds an internal option "status_only" to grep_opt,
which suppresses any output from grep_buffer().  Callers of the
function as library can use it to check if there is a match
without producing any output.

Signed-off-by: Junio C Hamano <junkio@cox.net>