perf hists browser: Add 'm' key for context menu display

With horizontal scrolling, the left/right arrow keys are used to scroll
columns and ENTER/ESC keys are used to enter/exit menu.  However if
callchain is recorded, the ENTER key is used to toggle callchain
expansion so there's no way to display menu.  Use 'm' key to display the
menu for this case.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1444694521-8136-1-git-send-email-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/ui/browsers/hists.c

@@ -1769,6 +1769,7 @@ static int perf_evsel__hists_browse(struct perf_evsel *evsel, int nr_events,
 	"E             Expand all callchains\n"				\
 	"F             Toggle percentage of filtered entries\n"		\
 	"H             Display column headers\n"			\
+	"m             Display context menu\n"				\
 	"S             Zoom into current Processor Socket\n"		\
 
 	/* help messages are sorted by lexical order of the hotkey */
@@ -1935,6 +1936,7 @@ static int perf_evsel__hists_browse(struct perf_evsel *evsel, int nr_events,
 			continue;
 		case K_ENTER:
 		case K_RIGHT:
+		case 'm':
 			/* menu */
 			break;
 		case K_ESC: