add merge highlight graphql file

android/Omnivore/app/src/main/graphql/MergeHighlight.graphql

@@ -0,0 +1,23 @@
+mutation MergeHighlight($input: MergeHighlightInput!) {
+  mergeHighlight(input: $input) {
+    ... on MergeHighlightSuccess {
+      highlight {
+        id
+        shortId
+        quote
+        prefix
+        suffix
+        patch
+        createdAt
+        updatedAt
+        annotation
+        sharedAt
+        createdByMe
+      }
+      overlapHighlightIdList
+    }
+    ... on MergeHighlightError {
+      errorCodes
+    }
+  }
+}