refactor(git): consolidate sync subprocess pattern via util.system_sync

lua/util.lua

@@ -441,4 +441,29 @@ 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