Add user preference for auto-selecting a base for merges

For reviewing merge commits most users expect to the see the full diff
that is integrated by accepting the merge commit. This diff is
available if the user selects 'Parent 1' from the 'Diff against'
drop-down list, however this is a new feature and most users don't
know about. Also it is inconvenient to manually select the first
parent for every merge. To make this easier add a user preferences for
automatically selecting a base for merges. Let the default for this
new preference be 'First Parent', as this is what most users expect.

Change-Id: I633291f78fee7f9ce1feb73deb81f9312363b347
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/Documentation/rest-api-accounts.txt b/Documentation/rest-api-accounts.txt
index 22545e3..13e5ca6 100644
--- a/Documentation/rest-api-accounts.txt
+++ b/Documentation/rest-api-accounts.txt
@@ -1213,6 +1213,7 @@
     "size_bar_in_change_table": true,
     "review_category_strategy": "ABBREV",
     "mute_common_path_prefixes": true,
+    "default_base_for_merges": "FIRST_PARENT",
     "my": [
       {
         "url": "#/dashboard/self",
@@ -2469,6 +2470,10 @@
 their own comments. On `DISABLED` the user will not receive any email
 notifications from Gerrit.
 Allowed values are `ENABLED`, `CC_ON_OWN_COMMENTS`, `DISABLED`.
+|`default_base_for_merges`      ||
+The base which should be pre-selected in the 'Diff Against' drop-down
+list when the change screen is opened for a merge commit.
+Allowed values are `AUTO_MERGE` and `FIRST_PARENT`.
 |============================================
 
 [[preferences-input]]
@@ -2527,6 +2532,10 @@
 their own comments. On `DISABLED` the user will not receive any email
 notifications from Gerrit.
 Allowed values are `ENABLED`, `CC_ON_OWN_COMMENTS`, `DISABLED`.
+|`default_base_for_merges`      |optional|
+The base which should be pre-selected in the 'Diff Against' drop-down
+list when the change screen is opened for a merge commit.
+Allowed values are `AUTO_MERGE` and `FIRST_PARENT`.
 |============================================
 
 [[query-limit-info]]