siso: fetch action_list shows action_digest by default

for format="", print action digest for action_list

before:
 % ~/go/bin/siso fetch -project rbe-android-ci -type action-list bc256fd3a797bee941b7cf71df1e7eb4047b90fc04a1cec4c2b80d3c9ecbce5a/157
command_digest:{hash:"cac87fd6ae77e58949e18422c88add162ffe6719bd57e70dba85d7d1cba39a7a" size_bytes:848} input_root_digest:{hash:"bebb012fc2efc9b5a6d1979c2ff4f9143c0950c21874ee84c1726cd0fa9d9128" size_bytes:78} timeout:{seconds:7200} salt:"tpc-exp.5"

after:
 % ~/go/bin/siso fetch -project rbe-android-ci -type action-list bc256fd3a797bee941b7cf71df1e7eb4047b90fc04a1cec4c2b80d3c9ecbce5a/157
action_digest: 4261b88a75d4644d3530756dbf161c63637d03d332614129e4510a3111572417/157
Change-Id: I9b33add11f5a2a6aab3d8dbbce84ab7ca1d27e16
Reviewed-on: https://chromium-review.googlesource.com/c/build/+/8257109
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Yuke Liao <liaoyuke@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>