Remove previousSha from GitHub PR events (#281)

* Remove previousSha from GitHUb PR events

* Fix tests

gradle-plugin/plugin/src/functionalTest/kotlin/com/emergetools/android/gradle/NoVcsEmergePluginTest.kt

@@ -100,7 +100,7 @@ class NoVcsEmergePluginTest : EmergePluginTest() {
 
     assertEquals("github_head_sha", configuration.vcsOptions!!.sha)
     assertEquals("github_base_sha", configuration.vcsOptions!!.baseSha)
-    assertEquals("github_previous_sha", configuration.vcsOptions!!.previousSha)
+    assertEquals(null, configuration.vcsOptions!!.previousSha)
     assertEquals("123", configuration.vcsOptions!!.prNumber)
   }
 
@@ -131,27 +131,4 @@ class NoVcsEmergePluginTest : EmergePluginTest() {
     // BaseSha not set by default
     assertNull(configuration.vcsOptions!!.baseSha)
   }
-
-  @Test
-  fun `Assert previousSha and baseSha are the same on first commit to PR`() {
-    val runner = EmergeGradleRunner.create("no-vcs-params")
-    val configurationJson = File(runner.tempProjectDir, "emerge_config.json")
-
-    runner
-      .withArguments("saveExtensionConfig", "--outputPath", configurationJson.path)
-      .withDebugTasks()
-      .withGitHubPREventNoBefore()
-      .assert { result, _ ->
-        result.assertSuccessfulTask(":saveExtensionConfig")
-      }
-      .build()
-
-    val configuration = Json.decodeFromStream<EmergePluginExtensionData>(
-      configurationJson.inputStream()
-    )
-
-    assertEquals("github_head_sha", configuration.vcsOptions!!.sha)
-    assertEquals("github_base_sha", configuration.vcsOptions!!.baseSha)
-    assertEquals("github_base_sha", configuration.vcsOptions!!.previousSha)
-  }
 }

gradle-plugin/plugin/src/main/kotlin/com/emergetools/android/gradle/util/GitHub.kt

@@ -70,9 +70,8 @@ internal class GitHub(private val execOperations: ExecOperations) {
   fun previousSha(): String? {
     return when {
       isPush() -> getPushEventData().before
-      // In the case of the first commit on a PR, the before sha is not available.
-      // We want previousSha to be the last commit on main, so we'll fallback to baseSha
-      isPullRequest() -> getPullRequestEventData().before ?: baseSha()
+      // In the case of branches, `before` can't be relied on from GitHub event data, so we'll skip
+      // previousSha for now on `pull_request` events
       else -> null
     }
   }
@@ -112,7 +111,6 @@ internal class GitHub(private val execOperations: ExecOperations) {
     @SerialName("pull_request")
     val pr: GitHubPullRequest,
     val number: Int,
-    val before: String? = null,
   )
 
   @Serializable