truncate PR description in backported git msg to five lines

sometimes these things are huge.

tools/backport_pr.py

@@ -1,171 +1,176 b''
 #!/usr/bin/env python
 """
 Backport pull requests to a particular branch.
 
 Usage: backport_pr.py branch [PR] [PR2]
 
 e.g.:
 
     python tools/backport_pr.py 0.13.1 123 155
 
 to backport PR #123 onto branch 0.13.1
 
 or
 
     python tools/backport_pr.py 2.1
 
 to see what PRs are marked for backport with milestone=2.1 that have yet to be applied
 to branch 2.x.
 
 """
 
 from __future__ import print_function
 
 import os
 import re
 import sys
 
 from subprocess import Popen, PIPE, check_call, check_output
 try:
     from urllib.request import urlopen
 except:
     from urllib import urlopen
 
 from gh_api import (
     get_issues_list,
     get_pull_request,
     get_pull_request_files,
     is_pull_request,
     get_milestone_id,
 )
 
 def find_rejects(root='.'):
     for dirname, dirs, files in os.walk(root):
         for fname in files:
             if fname.endswith('.rej'):
                 yield os.path.join(dirname, fname)
 
 def get_current_branch():
     branches = check_output(['git', 'branch'])
     for branch in branches.splitlines():
         if branch.startswith(b'*'):
             return branch[1:].strip().decode('utf-8')
 
 def backport_pr(branch, num, project='ipython/ipython'):
     current_branch = get_current_branch()
     if branch != current_branch:
         check_call(['git', 'checkout', branch])
     check_call(['git', 'pull'])
     pr = get_pull_request(project, num, auth=True)
     files = get_pull_request_files(project, num, auth=True)
     patch_url = pr['patch_url']
     title = pr['title']
     description = pr['body']
     fname = "PR%i.patch" % num
     if os.path.exists(fname):
         print("using patch from {fname}".format(**locals()))
         with open(fname) as f:
             patch = f.read()
     else:
         req = urlopen(patch_url)
         patch = req.read()
 
+    lines = description.splitlines()
+    if len(lines) > 5:
+        lines = lines[:5] + ['...']
+        description = '\n'.join(lines)
+
     msg = "Backport PR #%i: %s" % (num, title) + '\n\n' + description
     check = Popen(['git', 'apply', '--check', '--verbose'], stdin=PIPE)
     a,b = check.communicate(patch)
 
     if check.returncode:
         print("patch did not apply, saving to {fname}".format(**locals()))
         print("edit {fname} until `cat {fname} | git apply --check` succeeds".format(**locals()))
         print("then run tools/backport_pr.py {num} again".format(**locals()))
         if not os.path.exists(fname):
             with open(fname, 'wb') as f:
                 f.write(patch)
         return 1
 
     p = Popen(['git', 'apply'], stdin=PIPE)
     a,b = p.communicate(patch)
 
     filenames = [ f['filename'] for f in files ]
 
     check_call(['git', 'add'] + filenames)
 
     check_call(['git', 'commit', '-m', msg])
 
     print("PR #%i applied, with msg:" % num)
     print()
     print(msg)
     print()
 
     if branch != current_branch:
         check_call(['git', 'checkout', current_branch])
 
     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=None, milestone=None):
     """return set of PRs marked for backport"""
     if labels is None and milestone is None:
         raise ValueError("Specify one of labels or milestone.")
     elif labels is not None and milestone is not None:
         raise ValueError("Specify only one of labels or milestone.")
     if labels is not None:
         issues = get_issues_list("ipython/ipython",
                 labels=labels,
                 state='closed',
                 auth=True,
         )
     else:
         milestone_id = get_milestone_id("ipython/ipython", milestone,
                 auth=True)
         issues = get_issues_list("ipython/ipython",
                 milestone=milestone_id,
                 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
         if pr['base']['ref'] != 'master':
             continue
         should_backport.add(pr['number'])
     return should_backport
 
 if __name__ == '__main__':
 
     if len(sys.argv) < 2:
         print(__doc__)
         sys.exit(1)
 
     if len(sys.argv) < 3:
         milestone = sys.argv[1]
         branch = milestone.split('.')[0] + '.x'
         already = already_backported(branch)
         should = should_backport(milestone=milestone)
         print ("The following PRs should be backported:")
         for pr in sorted(should.difference(already)):
             print (pr)
         sys.exit(0)
 
     for prno in map(int, sys.argv[2:]):
         print("Backporting PR #%i" % prno)
         rc = backport_pr(sys.argv[1], prno)
         if rc:
             print("Backporting PR #%i failed" % prno)
             sys.exit(rc)