ARROW-6376: [Developer] Use target ref of PR when merging instead of …

…hard-coding "master"

I just used this successfully to merge ARROW-6314 into the ARROW-6313-flatbuffer-alignment branch

https://github.com/apache/arrow/tree/ARROW-6313-flatbuffer-alignment

Closes #5220 from wesm/ARROW-6376 and squashes the following commits:

54723c7 <Wes McKinney> Use target ref of PR when merging

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

dev/merge_arrow_pr.py

@@ -317,18 +317,18 @@ def _get_jira(self):
 
         return JiraIssue(self.con, jira_id, project, self.cmd)
 
-    def merge(self, target_ref='master'):
+    def merge(self):
         """
         merge the requested PR and return the merge hash
         """
         pr_branch_name = "%s_MERGE_PR_%s" % (BRANCH_PREFIX, self.number)
         target_branch_name = "%s_MERGE_PR_%s_%s" % (BRANCH_PREFIX,
                                                     self.number,
-                                                    target_ref.upper())
+                                                    self.target_ref.upper())
         run_cmd("git fetch %s pull/%s/head:%s" % (self.git_remote,
                                                   self.number,
                                                   pr_branch_name))
-        run_cmd("git fetch %s %s:%s" % (self.git_remote, target_ref,
+        run_cmd("git fetch %s %s:%s" % (self.git_remote, self.target_ref,
                                         target_branch_name))
         run_cmd("git checkout %s" % target_branch_name)
 
@@ -415,7 +415,7 @@ def merge(self, target_ref='master'):
         try:
             push_cmd = ('git push %s %s:%s' % (self.git_remote,
                                                target_branch_name,
-                                               target_ref))
+                                               self.target_ref))
             if DEBUG:
                 print(push_cmd)
             else: