oid: `const ZERO_SHA1` instead of `fn sha1_zero`

`fn sha1_zero` can be `const fn`, and since it does not take arguments,
it can just be a `const`. There is no good reason to not have a constant.

Justification for changing the order to first specify "zero" and then
"SHA1": In the future we will support multiple object formats. A
function that produces a zero OID would be parameterized by the object
format then. Imagine `fn zero(format: ObjectFormat)` which would be
called as `Oid::zero(ObjectForma::Sha1)`. In anticipation, we change the
ordering to match.

crates/radicle-cli/src/git/unified_diff.rs

@@ -336,7 +336,7 @@ impl Encode for FileHeader {
                 w.meta(format!("new file mode {:o}", u32::from(new.mode.clone())))?;
                 w.meta(format!(
                     "index {}..{}",
-                    term::format::oid(git::Oid::sha1_zero()),
+                    term::format::oid(git::Oid::ZERO_SHA1),
                     term::format::oid(*new.oid),
                 ))?;
 
@@ -358,7 +358,7 @@ impl Encode for FileHeader {
                 w.meta(format!(
                     "index {}..{}",
                     term::format::oid(*old.oid),
-                    term::format::oid(git::Oid::sha1_zero())
+                    term::format::oid(git::Oid::ZERO_SHA1)
                 ))?;
 
                 w.meta(format!("--- a/{}", path.display()))?;

crates/radicle-node/src/test/handle.rs

@@ -118,7 +118,7 @@ impl radicle::node::Handle for Handle {
 
         Ok(RefsAt {
             remote: self.nid()?,
-            at: Oid::sha1_zero(),
+            at: Oid::ZERO_SHA1,
         })
     }
 

crates/radicle-oid/src/lib.rs

@@ -87,6 +87,8 @@ pub enum Oid {
 // for forwards compatibility: What if another hash with digests of the same
 // length becomes popular?
 impl Oid {
+    pub const ZERO_SHA1: Self = Self::Sha1([0u8; SHA1_DIGEST_LEN]);
+
     pub fn from_sha1(digest: [u8; SHA1_DIGEST_LEN]) -> Self {
         Self::Sha1(digest)
     }
@@ -96,10 +98,6 @@ impl Oid {
             Oid::Sha1(digest) => Some(*digest),
         }
     }
-
-    pub fn sha1_zero() -> Self {
-        Self::Sha1([0u8; SHA1_DIGEST_LEN])
-    }
 }
 
 /// Interaction with zero.
@@ -230,7 +228,7 @@ pub mod str {
                 "0000000000000000000000000000000000000000"
                     .parse::<Oid>()
                     .unwrap(),
-                Oid::sha1_zero()
+                Oid::ZERO_SHA1
             );
         }
 
@@ -319,7 +317,7 @@ mod fmt {
         #[test]
         fn zero() {
             assert_eq!(
-                Oid::sha1_zero().to_string(),
+                Oid::ZERO_SHA1.to_string(),
                 "0000000000000000000000000000000000000000"
             );
         }
@@ -404,7 +402,7 @@ mod gix {
 
         #[test]
         fn zero() {
-            assert!(Oid::sha1_zero() == Other::null(Kind::Sha1));
+            assert!(Oid::ZERO_SHA1 == Other::null(Kind::Sha1));
         }
     }
 }
@@ -451,7 +449,7 @@ mod git2 {
 
         #[test]
         fn zero() {
-            assert!(Oid::sha1_zero() == Other::zero());
+            assert!(Oid::ZERO_SHA1 == Other::zero());
         }
     }
 }

crates/radicle/src/storage/refs.rs

@@ -177,7 +177,7 @@ impl Refs {
         let mut buf = String::new();
 
         for (name, oid) in self.0.iter() {
-            debug_assert_ne!(oid, &Oid::sha1_zero());
+            debug_assert_ne!(oid, &Oid::ZERO_SHA1);
             debug_assert_ne!(name, &SIGREFS_BRANCH.to_ref_string());
 
             buf.push_str(&oid.to_string());