refactor(git): make the git module self-contained under git.util

lua/util.lua

@@ -441,29 +441,4 @@ function M.split_lines(content)
     return lines
 end
 
----Run a system command synchronously and return stdout on success. On
----non-zero exit, logs stderr via `log.error` and returns nil. Pass
----`opts.silent` to suppress the auto-log when failure is expected (e.g.
----probe-style commands like `git rev-parse` against a possibly-missing
----ref).
----@param cmd string[]
----@param opts { cwd: string?, stdin: string?, silent: boolean? }?
----@return string?
-function M.system_sync(cmd, opts)
-    opts = opts or {}
-    local result = vim.system(cmd, {
-        cwd = opts.cwd,
-        stdin = opts.stdin,
-        text = true,
-    }):wait()
-    if result.code ~= 0 then
-        if not opts.silent then
-            local label = cmd[2] and (cmd[1] .. " " .. cmd[2]) or cmd[1] or "?"
-            log.error("%s failed: %s", label, vim.trim(result.stderr or ""))
-        end
-        return nil
-    end
-    return result.stdout or ""
-end
-
 return M