survey: clearly note the experimental nature in the output

While this command is definitely something we _want_, chances are that
upstreaming this will require substantial changes.

We still want to be able to experiment with this before that, to focus
on what we need out of this command: To assist with diagnosing issues
with large repositories, as well as to help monitoring the growth and
the associated painpoints of such repositories.

To that end, we are about to integrate this command into
`microsoft/git`, to get the tool into the hands of users who need it
most, with the idea to iterate in close collaboration between these
users and the developers familar with Git's internals.

However, we will definitely want to avoid letting anybody have the
impression that this command, its exact inner workings, as well as its
output format, are anywhere close to stable. To make that fact utterly
clear (and thereby protect the freedom to iterate and innovate freely
before upstreaming the command), let's mark its output as experimental
in all-caps, as the first thing we do.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/survey.c

@@ -17,6 +17,7 @@
 #include "strvec.h"
 #include "tag.h"
 #include "trace2.h"
+#include "color.h"
 
 static const char * const survey_usage[] = {
 	N_("(EXPERIMENTAL!) 'git survey' <options>"),
@@ -901,6 +902,11 @@ int cmd_survey(int argc, const char **argv, const char *prefix, struct repositor
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage_with_options(survey_usage, survey_options);
 
+	if (isatty(2))
+		color_fprintf_ln(stderr,
+				 want_color_fd(2, GIT_COLOR_AUTO) ? GIT_COLOR_YELLOW : "",
+				 "(THIS IS EXPERIMENTAL, EXPECT THE OUTPUT FORMAT TO CHANGE!)");
+
 	ctx.repo = repo;
 
 	prepare_repo_settings(ctx.repo);