fsmonitor-settings: NTFS and FAT32 on MacOS are incompatible

On MacOS mark repos on NTFS or FAT32 volumes as incompatible.

The builtin FSMonitor used Unix domain sockets on MacOS for IPC
with clients.  These sockets are kept in the .git directory.
Unix sockets are not supported by NTFS and FAT32, so the daemon
cannot start up.

Test for this during our compatibility checking so that client
commands do not keep trying to start the daemon.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/fsmonitor-settings.c b/fsmonitor-settings.c
index d2fb014..658cb79 100644
--- a/fsmonitor-settings.c
+++ b/fsmonitor-settings.c
@@ -225,6 +225,12 @@ char *fsm_settings__get_incompatible_msg(const struct repository *r,
 			    _("virtual repository '%s' is incompatible with fsmonitor"),
 			    r->worktree);
 		goto done;
+
+	case FSMONITOR_REASON_NOSOCKETS:
+		strbuf_addf(&msg,
+			    _("repository '%s' is incompatible with fsmonitor due to lack of Unix sockets"),
+			    r->worktree);
+		goto done;
 	}
 
 	BUG("Unhandled case in fsm_settings__get_incompatible_msg: '%d'",