Enable the built-in FSMonitor as an experimental feature

If `feature.experimental` and `feature.manyFiles` are set and the user
has not explicitly turned off the builtin FSMonitor, we now start
the built-in FSMonitor by default.

Only forcing it when UNSET matches the behavior of UPDATE_DEFAULT_BOOL()
used for other repo settings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>

repo-settings.c

@@ -2,6 +2,8 @@
 #include "config.h"
 #include "repository.h"
 #include "midx.h"
+#include "fsmonitor-ipc.h"
+#include "fsmonitor-settings.h"
 
 static void repo_cfg_bool(struct repository *r, const char *key, int *dest,
 			  int def)
@@ -45,6 +47,30 @@ void prepare_repo_settings(struct repository *r)
 		r->settings.fetch_negotiation_algorithm = FETCH_NEGOTIATION_SKIPPING;
 		r->settings.pack_use_bitmap_boundary_traversal = 1;
 		r->settings.pack_use_multi_pack_reuse = 1;
+
+		/*
+		 * Force enable the builtin FSMonitor (unless the repo
+		 * is incompatible or they've already selected it or
+		 * the hook version).  But only if they haven't
+		 * explicitly turned it off -- so only if our config
+		 * value is UNSET.
+		 *
+		 * lookup_fsmonitor_settings() and check_for_ipc() do
+		 * not distinguish between explicitly set FALSE and
+		 * UNSET, so we re-test for an UNSET config key here.
+		 *
+		 * I'm not sure I want to fix fsmonitor-settings.c to
+		 * have more than one _DISABLED state since our usage
+		 * here is only to support this experimental period
+		 * (and I don't want to overload the _reason field
+		 * because it describes incompabilities).
+		 */
+		if (manyfiles &&
+		    fsmonitor_ipc__is_supported()  &&
+		    fsm_settings__get_mode(r) == FSMONITOR_MODE_DISABLED &&
+		    repo_config_get_maybe_bool(r, "core.fsmonitor", &value) > 0 &&
+		    repo_config_get_bool(r, "core.useBuiltinFSMonitor", &value))
+			fsm_settings__set_ipc(r);
 	}
 	if (manyfiles) {
 		r->settings.index_version = 4;