add ability to check what PRs should be backported in backport_pr

just run:

    python tools/backport_pr.py 1.x

to see what PRs are marked for backport, but not yet applied.

tools/backport_pr.py

@@ -2,24 +2,37 b''
 """
 Backport pull requests to a particular branch.
 
-Usage: backport_pr.py branch PR
+Usage: backport_pr.py branch [PR]
 
 e.g.:
 
-    backport_pr.py 0.13.1 123
+    python tools/backport_pr.py 0.13.1 123
 
 to backport PR #123 onto branch 0.13.1
 
+or
+
+    python tools/backport_pr.py 1.x
+
+to see what PRs are marked for backport that have yet to be applied.
+
 """
 
 from __future__ import print_function
 
 import os
+import re
 import sys
+
 from subprocess import Popen, PIPE, check_call, check_output
 from urllib import urlopen
 
-from gh_api import get_pull_request, get_pull_request_files
+from gh_api import (
+    get_issues_list,
+    get_pull_request,
+    get_pull_request_files,
+    is_pull_request,
+)
 
 def find_rejects(root='.'):
     for dirname, dirs, files in os.walk(root):
@@ -84,9 +97,47 b" def backport_pr(branch, num, project='ipython/ipython'):"
     
     return 0
 
+backport_re = re.compile(r"[Bb]ackport.*?(\d+)")
+
+def already_backported(branch, since_tag=None):
+    """return set of PRs that have been backported already"""
+    if since_tag is None:
+        since_tag = check_output(['git','describe', branch, '--abbrev=0']).decode('utf8').strip()
+    cmd = ['git', 'log', '%s..%s' % (since_tag, branch), '--oneline']
+    lines = check_output(cmd).decode('utf8')
+    return set(int(num) for num in backport_re.findall(lines))
+
+def should_backport(labels):
+    """return set of PRs marked for backport"""
+    issues = get_issues_list("ipython/ipython",
+            labels=labels,
+            state='closed',
+            auth=True,
+    )
+    should_backport = set()
+    for issue in issues:
+        if not is_pull_request(issue):
+            continue
+        pr = get_pull_request("ipython/ipython", issue['number'], auth=True)
+        if not pr['merged']:
+            print ("Marked PR closed without merge: %i" % pr['number'])
+            continue
+        should_backport.add(pr['number'])
+    return should_backport
+
 if __name__ == '__main__':
-    if len(sys.argv) < 3:
+    
+    if len(sys.argv) < 2:
         print(__doc__)
         sys.exit(1)
     
+    if len(sys.argv) < 3:
+        branch = sys.argv[1]
+        already = already_backported(branch)
+        should = should_backport("backport-1.1")
+        print ("The following PRs should be backported:")
+        for pr in should.difference(already):
+            print (pr)
+        sys.exit(0)
+    
     sys.exit(backport_pr(sys.argv[1], int(sys.argv[2])))