refactor(git): unify diff/object dispatch, codify naming, add :Gdiffsplit

lua/git/repo.lua

@@ -343,4 +343,18 @@ function M.rev_parse(worktree, ref, short)
     return trimmed ~= "" and trimmed or nil
 end
 
+---Verify a revspec resolves to an existing git object. `cat-file -e` is
+---git's cheapest existence check, and unlike `rev-parse --verify` it
+---also accepts the `<commit>:<path>` form that BufReadCmd revspec URIs
+---use.
+---@param worktree string
+---@param revspec string
+---@return boolean
+function M.object_exists(worktree, revspec)
+    return util.exec(
+        { "git", "cat-file", "-e", revspec },
+        { cwd = worktree, silent = true }
+    ) ~= nil
+end
+
 return M