Let landmines print something when run as part of `gclient sync`.

(Maybe only needed on Windows.)

Landmines take unusably long when they run on Windows, and if this
prints the diff I can ctrl-c, manually apply that diff to
.landmines, and then runhooks again :-/

BUG=none

Review-Url: https://codereview.chromium.org/1947523002
Cr-Original-Commit-Position: refs/heads/master@{#391291}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d7d6455c81e282c92daff0f396ea55fbc5d22235

landmines.py

@@ -73,6 +73,7 @@ def clobber_if_necessary(new_landmines, src_dir):
           fromfiledate=old_date, tofiledate=time.ctime(), n=0)
       sys.stdout.write('Clobbering due to:\n')
       sys.stdout.writelines(diff)
+      sys.stdout.flush()
 
       clobber.clobber(out_dir)