contrib: deterministic-fuzz-coverage fixups

* Name the fuzz_corpora dir after its real name.
* Add missing cargo lock file.
* Use git instead of diff command to increase compatibility
* Use --help instead of --version to increase compatibility
* Use assert consistently for unexpected errors.
* Remove redundant Stdio::from.
* Fix typos.

contrib/devtools/deterministic-fuzz-coverage/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "deterministic-fuzz-coverage"
+version = "0.1.0"