(ipld-prime) Use mh.Set() for "hash" -> mhType & Make git plugin test work again

[rvagg]

@mvdan this extends your last commit to the branch to do mh.Set() for hash as well.

It also revives t0280 for the git-plugin. The diff with master is the most interesting bit now:

diff --git a/test/sharness/t0280-plugin-git.sh b/test/sharness/t0280-plugin-git.sh
index e7bc06427..d90df46bc 100755
--- a/test/sharness/t0280-plugin-git.sh
+++ b/test/sharness/t0280-plugin-git.sh
@@ -17,7 +17,7 @@ test_expect_success "prepare test data" '

 test_dag_git() {
   test_expect_success "add objects via dag put" '
-    find objects -type f -exec ipfs dag put --format=git --input-enc=zlib {} \; -exec echo \; > hashes
+    find objects -type f -exec ipfs dag put --format=git-raw --input-enc=3145848 --hash=sha1 {} \; -exec echo -n \; > hashes
   '

   test_expect_success "successfully get added objects" '
@@ -25,8 +25,8 @@ test_dag_git() {
   '

   test_expect_success "path traversals work" '
-    echo \"YmxvYiA3ACcsLnB5Zgo=\" > file1 &&
-    ipfs dag get baf4bcfhzi72pcj5cc4ocz7igcduubuu7aa3cddi/object/parents/0/tree/dir2/hash/f3/hash > out1
+    echo -n "{\"/\":{\"bytes\":\"YmxvYiA3ACcsLnB5Zgo=\"}}" > file1 &&
+    ipfs dag get baf4bcfhzi72pcj5cc4ocz7igcduubuu7aa3cddi/Object/Parents/0/GitTree/1/Hash/0/Hash > out1
   '

   test_expect_success "outputs look correct" '

Note the pathing through the git object at the bottom there. I wouldn't mind getting that addressed - don't CamelCase, at least one rename, and maybe turn those arrays into maps so we can do it by name. It'd be nice to maintain consistency of forms.

[rvagg]

Noice, Circle giving green now for sharness