Split up read-cache.c into more logical clumps.

Do the usage and error reporting in "usage.c", and the sha1 file
accesses in "sha1_file.c".

Small, nice, easily separated parts. Good.

usage.c

@@ -0,0 +1,40 @@
+/*
+ * GIT - The information manager from hell
+ *
+ * Copyright (C) Linus Torvalds, 2005
+ */
+#include <stdarg.h>
+#include "cache.h"
+
+static void report(const char *prefix, const char *err, va_list params)
+{
+	fputs(prefix, stderr);
+	vfprintf(stderr, err, params);
+	fputs("\n", stderr);
+}
+
+void usage(const char *err)
+{
+	fprintf(stderr, "usage: %s\n", err);
+	exit(1);
+}
+
+void die(const char *err, ...)
+{
+	va_list params;
+
+	va_start(params, err);
+	report("fatal: ", err, params);
+	va_end(params);
+	exit(1);
+}
+
+int error(const char *err, ...)
+{
+	va_list params;
+
+	va_start(params, err);
+	report("error: ", err, params);
+	va_end(params);
+	return -1;
+}