Mimic git merge-base

Github doesn't provide a merge-base thing, so previously the code used
"master" to compare against, which can easily fail.

Find a better commit to compare config contents against (config/owner
checks) using the list of commits in the PR.

Note that an alternative would be to use GH's `potentialMergeCommit`
(compare "master" as the old, and this as the new).  It takes a while
for it to be generated, but we already wait until `mergeable` is not
"UNKNOWN" which might mean that this is ready too.  But even with that,
it seems like a bad idea to rely on information from these temporary
merge branches.

Author: elibarzilay

src/_tests/fixtures/38979/_response.json

@@ -38,6 +38,7 @@
         "state": "OPEN",
         "headRefOid": "222334139e52fc16369464cfb5dc95c82f71192f",
         "changedFiles": 72,
+        "commitIds": {},
         "timelineItems": {
           "nodes": [
             {

src/_tests/fixtures/38979/derived.json

@@ -4,6 +4,7 @@
   "pr_number": 38979,
   "author": "ExE-Boss",
   "headCommitOid": "222334139e52fc16369464cfb5dc95c82f71192f",
+  "mergeBaseOid": "master",
   "lastPushDate": "2020-01-03T21:02:41.000Z",
   "lastActivityDate": "2020-05-12T15:13:19.000Z",
   "hasMergeConflict": false,

src/_tests/fixtures/43136/_response.json

@@ -37,6 +37,7 @@
         "state": "OPEN",
         "headRefOid": "e6863537248bbfee8f0ef8c636bb00c25cf40b96",
         "changedFiles": 2,
+        "commitIds": {},
         "timelineItems": {
           "nodes": [
             {

src/_tests/fixtures/43136/derived.json

@@ -4,6 +4,7 @@
   "pr_number": 43136,
   "author": "larsrh",
   "headCommitOid": "e6863537248bbfee8f0ef8c636bb00c25cf40b96",
+  "mergeBaseOid": "master",
   "lastPushDate": "2020-03-16T14:26:42.000Z",
   "lastActivityDate": "2020-03-16T14:31:40.000Z",
   "hasMergeConflict": false,

src/_tests/fixtures/43144/_response.json

@@ -33,6 +33,7 @@
         "state": "OPEN",
         "headRefOid": "f1f5c4bb0ae553f56766882f6458d2e22baa87c7",
         "changedFiles": 2,
+        "commitIds": {},
         "timelineItems": {
           "nodes": [
             {

src/_tests/fixtures/43144/derived.json

@@ -4,6 +4,7 @@
   "pr_number": 43144,
   "author": "jeffreymeng",
   "headCommitOid": "f1f5c4bb0ae553f56766882f6458d2e22baa87c7",
+  "mergeBaseOid": "master",
   "lastPushDate": "2020-03-15T00:11:48.000Z",
   "lastActivityDate": "2020-03-15T01:29:09.000Z",
   "hasMergeConflict": false,

src/_tests/fixtures/43151/_response.json

@@ -37,6 +37,7 @@
         "state": "OPEN",
         "headRefOid": "bb6d3150b485cd203d265e06ca910262256e523e",
         "changedFiles": 4,
+        "commitIds": {},
         "timelineItems": {
           "nodes": [
             {

src/_tests/fixtures/43151/derived.json

@@ -4,6 +4,7 @@
   "pr_number": 43151,
   "author": "adamzerella",
   "headCommitOid": "bb6d3150b485cd203d265e06ca910262256e523e",
+  "mergeBaseOid": "master",
   "lastPushDate": "2020-03-15T12:50:20.000Z",
   "lastActivityDate": "2020-03-15T18:11:57.000Z",
   "hasMergeConflict": false,

src/_tests/fixtures/43160/_response.json

@@ -37,6 +37,7 @@
         "state": "OPEN",
         "headRefOid": "6d5d2a85b41d287f97c9d331a9ff6a9824e2f1ff",
         "changedFiles": 1,
+        "commitIds": {},
         "timelineItems": {
           "nodes": [
             {

src/_tests/fixtures/43160/derived.json

@@ -4,6 +4,7 @@
   "pr_number": 43160,
   "author": "rikkertkoppes",
   "headCommitOid": "6d5d2a85b41d287f97c9d331a9ff6a9824e2f1ff",
+  "mergeBaseOid": "master",
   "lastPushDate": "2020-03-16T08:14:50.000Z",
   "lastActivityDate": "2020-03-17T14:33:47.000Z",
   "hasMergeConflict": false,