use check_call for git add and commit

in backport_pr

tools/backport_pr.py

@@ -70,18 +70,10 b" def backport_pr(branch, num, project='ipython/ipython'):"
     
     filenames = [ f['filename'] for f in files ]
 
-    add = Popen(['git', 'add'] + filenames)
-    add.wait()
-    if add.returncode:
-        print("add failed!")
-        return 1
+    check_call(['git', 'add'] + filenames)
+    
+    check_call(['git', 'commit', '-m', msg])
 
-    commit = Popen(['git', 'commit', '-m', msg])
-    commit.wait()
-    if commit.returncode:
-        print("commit failed!")
-        return 1
-    else:
         print("PR #%i applied, with msg:" % num)
         print()
         print(msg)