Use fetch instead of cherry-pick for Gerrit changes

This enables support for dependent changes.

Fixed: 182
Change-Id: I962e19b5ee6dffec0c2044abb671edf0438b1418
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/10180
Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Petr Hosek <phosek@google.com>

infra/recipes/gn.expected/cq_linux.json

diff --git a/infra/recipes/gn.expected/cq_linux.json b/infra/recipes/gn.expected/cq_linux.json
index 1dfdf03..a309a56 100644
--- a/infra/recipes/gn.expected/cq_linux.json
+++ b/infra/recipes/gn.expected/cq_linux.json
@@ -74,12 +74,12 @@
   {
     "cmd": [
       "git",
-      "cherry-pick",
+      "checkout",
       "FETCH_HEAD"
     ],
     "cwd": "[START_DIR]/gn",
     "infra_step": true,
-    "name": "git.cherry-pick 123456/7",
+    "name": "git.checkout 123456/7",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]

infra/recipes/gn.expected/cq_mac.json

diff --git a/infra/recipes/gn.expected/cq_mac.json b/infra/recipes/gn.expected/cq_mac.json
index 3741621..e7c477a 100644
--- a/infra/recipes/gn.expected/cq_mac.json
+++ b/infra/recipes/gn.expected/cq_mac.json
@@ -74,12 +74,12 @@
   {
     "cmd": [
       "git",
-      "cherry-pick",
+      "checkout",
       "FETCH_HEAD"
     ],
     "cwd": "[START_DIR]/gn",
     "infra_step": true,
-    "name": "git.cherry-pick 123456/7",
+    "name": "git.checkout 123456/7",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]

infra/recipes/gn.expected/cq_win.json

diff --git a/infra/recipes/gn.expected/cq_win.json b/infra/recipes/gn.expected/cq_win.json
index 8e352de..164b245 100644
--- a/infra/recipes/gn.expected/cq_win.json
+++ b/infra/recipes/gn.expected/cq_win.json
@@ -74,12 +74,12 @@
   {
     "cmd": [
       "git",
-      "cherry-pick",
+      "checkout",
       "FETCH_HEAD"
     ],
     "cwd": "[START_DIR]\\gn",
     "infra_step": true,
-    "name": "git.cherry-pick 123456/7",
+    "name": "git.checkout 123456/7",
     "~followup_annotations": [
       "@@@STEP_NEST_LEVEL@1@@@"
     ]

infra/recipes/gn.py

diff --git a/infra/recipes/gn.py b/infra/recipes/gn.py
index 7f4602a..89dc8ee 100644
--- a/infra/recipes/gn.py
+++ b/infra/recipes/gn.py
@@ -50,8 +50,8 @@
             'refs/changes/%s/%s/%s' %
             (str(change.change)[-2:], change.change, change.patchset)
         ])
-        api.step('cherry-pick %s/%s' % (change.change, change.patchset),
-                 ['git', 'cherry-pick', 'FETCH_HEAD'])
+        api.step('checkout %s/%s' % (change.change, change.patchset),
+                 ['git', 'checkout', 'FETCH_HEAD'])
 
   with api.context(infra_steps=True):
     cipd_dir = api.path['start_dir'].join('cipd')