ARROW-13: Add PR merge tool from parquet-mr, suitably modified

dev/README.md

@@ -0,0 +1,94 @@
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+
+# Arrow Developer Scripts
+
+This directory contains scripts useful to developers when packaging,
+testing, or committing to Arrow.
+
+Merging a pull request requires being a committer on the project.
+
+* How to merge a Pull request:
+have an apache and apache-github remote setup
+```
+git remote add apache-github https://github.com/apache/arrow.git
+git remote add apache https://git-wip-us.apache.org/repos/asf/arrow.git
+```
+run the following command
+```
+dev/merge_arrow_pr.py
+```
+
+Note:
+* The directory name of your Arrow git clone must be called arrow
+* Without jira-python installed you'll have to close the JIRA manually
+
+example output:
+```
+Which pull request would you like to merge? (e.g. 34):
+```
+Type the pull request number (from https://github.com/apache/arrow/pulls) and hit enter.
+```
+=== Pull Request #X ===
+title	Blah Blah Blah
+source	repo/branch
+target	master
+url	https://api.github.com/repos/apache/arrow/pulls/X
+
+Proceed with merging pull request #3? (y/n):
+```
+If this looks good, type y and hit enter.
+```
+From git-wip-us.apache.org:/repos/asf/arrow.git
+ * [new branch]      master     -> PR_TOOL_MERGE_PR_3_MASTER
+Switched to branch 'PR_TOOL_MERGE_PR_3_MASTER'
+
+Merge complete (local ref PR_TOOL_MERGE_PR_3_MASTER). Push to apache? (y/n):
+```
+A local branch with the merge has been created.
+type y and hit enter to push it to apache master
+```
+Counting objects: 67, done.
+Delta compression using up to 4 threads.
+Compressing objects: 100% (26/26), done.
+Writing objects: 100% (36/36), 5.32 KiB, done.
+Total 36 (delta 17), reused 0 (delta 0)
+To git-wip-us.apache.org:/repos/arrow-mr.git
+   b767ac4..485658a  PR_TOOL_MERGE_PR_X_MASTER -> master
+Restoring head pointer to b767ac4e
+Note: checking out 'b767ac4e'.
+
+You are in 'detached HEAD' state. You can look around, make experimental
+changes and commit them, and you can discard any commits you make in this
+state without impacting any branches by performing another checkout.
+
+If you want to create a new branch to retain commits you create, you may
+do so (now or later) by using -b with the checkout command again. Example:
+
+  git checkout -b new_branch_name
+
+HEAD is now at b767ac4... Update README.md
+Deleting local branch PR_TOOL_MERGE_PR_X
+Deleting local branch PR_TOOL_MERGE_PR_X_MASTER
+Pull request #X merged!
+Merge hash: 485658a5
+
+Would you like to pick 485658a5 into another branch? (y/n):
+```
+For now just say n as we have 1 branch