prune: Reduce memory allocations while repacking

The slicing operator `slice[low:high]` default to 0 for the lower bound and
len(slice) for the upper bound when either or both are not specified.
Fix the code to use `cap(slice)` to check for the slice capacity.

internal/repository/repack.go

@@ -57,8 +57,7 @@ func Repack(ctx context.Context, repo restic.Repository, packs restic.IDSet, kee
 
 			debug.Log("  process blob %v", h)
 
-			buf = buf[:]
-			if uint(len(buf)) < entry.Length {
+			if uint(cap(buf)) < entry.Length {
 				buf = make([]byte, entry.Length)
 			}
 			buf = buf[:entry.Length]