Use hashcpy() when copying object names

Use hashcpy() when copying object names

We invented hashcpy() to keep the abstraction of "object name"
behind it.  Use it instead of calling memcpy() with hard-coded
20-byte length when moving object names between pieces of memory.

Leave ppc/sha1.c as-is, because the function is about the SHA-1 hash
algorithm whose output is and will always be 20 bytes.

Helped-by: Michael Haggerty <redacted>
Helped-by: Duy Nguyen <redacted>
Signed-off-by: Sun He <redacted>
Signed-off-by: Junio C Hamano <redacted>