git-metadata: Add `CommitData::strip_signatures`

To allow for the verification of a commit signature, the signature
needs to be stripped before converting the commit data to bytes.

crates/radicle-git-metadata/src/commit.rs

@@ -85,6 +85,11 @@ impl<Tree, Parent> CommitData<Tree, Parent> {
         self.headers.signatures()
     }
 
+    pub fn strip_signatures(mut self) -> Self {
+        self.headers.strip_signatures();
+        self
+    }
+
     /// The [`Headers`] found in this commit.
     ///
     /// Note: these do not include `tree`, `parent`, `author`, and `committer`.

crates/radicle-git-metadata/src/commit/headers.rs

@@ -70,6 +70,10 @@ impl Headers {
     pub fn push(&mut self, name: &str, value: &str) {
         self.0.push((name.to_owned(), value.trim().to_owned()));
     }
+
+    pub(crate) fn strip_signatures(&mut self) {
+        self.0.retain(|(key, _)| key != "gpgsig");
+    }
 }
 
 #[derive(Debug, thiserror::Error)]