internal/fuzz: write shorter testdata corpus file names

The only purpose of using the SHA256 in the file name is
collision avoidance. Using just the first 64 bits (16 hex digits)
will be more than enough, unless people start storing billions
of test cases in their corpora.

The shorter names are nicer for just about everything:
command lines, repository listings, and so on.

Change-Id: I67c760023bed85ba3ffd4f8058f86ef778322ba7
Reviewed-on: https://go-review.googlesource.com/c/go/+/443335
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
Reviewed-by: Peter Weinberger <[email protected]>
Run-TryBot: Russ Cox <[email protected]>

Author: rsc

src/internal/fuzz/fuzz.go

@@ -1032,7 +1032,7 @@ func CheckCorpus(vals []any, types []reflect.Type) error {
 // writeToCorpus will not rewrite it. writeToCorpus sets entry.Path to the new
 // file that was just written or an error if it failed.
 func writeToCorpus(entry *CorpusEntry, dir string) (err error) {
-	sum := fmt.Sprintf("%x", sha256.Sum256(entry.Data))
+	sum := fmt.Sprintf("%x", sha256.Sum256(entry.Data))[:16]
 	entry.Path = filepath.Join(dir, sum)
 	if err := os.MkdirAll(dir, 0777); err != nil {
 		return err