jobd/gc.sh: switch the gc "signature" hash to use ltsha256

jobd/gc.sh: switch the gc "signature" hash to use ltsha256

When generating the final "repack" "signature" hash, ALL of the
existing ref logs are fed into the hash function.

Normally there aren't any ref logs.  However, if any worktrees
exist or the repository has been made not-bare, there very well
could be.  A repository with a lot of ongoing development activity
could potentially have a lot of very long files in the "logs"
subdirectory.

Sucking all of them plus the full packed-refs file into memory
at once shouldn't be a problem...   There really should always
be enough memory to do this...  But, just in case...

Avoid any unnecessary memory bloat (as well as an outdated SHA-1
algorithm) and switch all of the gc "signature" hashes from
using `git hash-object -t blob --stdin` to `ltsha256`.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>