fix libgit2 error throw. Fixes #42575 (#42576)

(cherry picked from commit 7a8a39b)

Author: IanButterworth

stdlib/LibGit2/src/rebase.jl

@@ -83,7 +83,7 @@ function commit(rb::GitRebase, sig::GitSignature)
                       oid_ptr, rb.ptr, C_NULL, sig.ptr, C_NULL, C_NULL)
     catch err
         # TODO: return current HEAD instead
-        err.code === Error.EAPPLIED && return nothing
+        err isa GitError && err.code === Error.EAPPLIED && return nothing
         rethrow()
     end
     return oid_ptr[]