chore: forbid branch and tag for git source

Author: remimimimimi

crates/pixi_spec/src/toml.rs

@@ -557,9 +557,10 @@ impl<'de> toml_span::Deserialize<'de> for TomlSpec {
             .optional::<TomlFromStr<_>>("git")
             .map(TomlFromStr::into_inner);
         let path = th.optional("path");
-        let branch = th.optional("branch");
         let rev = th.optional("rev");
-        let tag = th.optional("tag");
+        // TODO: Uncomment when lockfile source pinning is added.
+        // let branch = th.optional("branch");
+        // let tag = th.optional("tag");
         let subdirectory = th.optional("subdirectory");
         let build = th
             .optional::<TomlFromStr<_>>("build")
@@ -586,9 +587,9 @@ impl<'de> toml_span::Deserialize<'de> for TomlSpec {
                 url,
                 git,
                 path,
-                branch,
                 rev,
-                tag,
+                branch: None,
+                tag: None,
                 subdirectory,
                 md5,
                 sha256,