Use pylint 2.7 for depot_tools

This includes a few fixes for specific errors, and disables several new
warnings introduced in this version, in order to allow for an incremental migration.

Bug:1262286
Change-Id: Ie97d686748c9c952e87718a65f401c5f6f80a5c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3400616
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>

git_common.py

@@ -46,6 +46,7 @@ from io import BytesIO
 
 if sys.version_info.major == 2:
   # On Python 3, BrokenPipeError is raised instead.
+  # pylint:disable=redefined-builtin
   BrokenPipeError = IOError
 
 
@@ -874,7 +875,7 @@ def status():
     while c != b'':
       c = stream.read(1)
       if c in (None, b'', b'\0'):
-        if len(acc.getvalue()):
+        if len(acc.getvalue()) > 0:
           yield acc.getvalue()
           acc = BytesIO()
       else: