grep: teach --debug option to dump the parse tree

Our "grep" allows complex boolean expressions to be formed to match
each individual line with operators like --and, '(', ')' and --not.
Introduce the "--debug" option to show the parse tree to help people
who want to debug and enhance it.

Also "log" learns "--grep-debug" option to do the same.  The command
line parser to the log family is a lot more limited than the general
"git grep" parser, but it has special handling for header matching
(e.g. "--author"), and a parse tree is valuable when working on it.

Note that "--all-match" is *not* any individual node in the parse
tree.  It is an instruction to the evaluator to check all the nodes
in the top-level backbone have matched and reject a document as
non-matching otherwise.

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

builtin/grep.c

@@ -772,6 +772,9 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			   "indicate hit with exit status without output"),
 		OPT_BOOLEAN(0, "all-match", &opt.all_match,
 			"show only matches from files that match all patterns"),
+		{ OPTION_SET_INT, 0, "debug", &opt.debug, NULL,
+		  "show parse tree for grep expression",
+		  PARSE_OPT_NOARG | PARSE_OPT_HIDDEN, NULL, 1 },
 		OPT_GROUP(""),
 		{ OPTION_STRING, 'O', "open-files-in-pager", &show_in_pager,
 			"pager", "show matching files in the pager",