substitute surrogates in log file (#8076)

Summary:
Pull Request resolved: facebookincubator/below#8076

D26025779 (5ab476f) may have broken the getdeps build when logging
surrogates. Use errors=surrogateescape to try to avoid that.

Reviewed By: danobi

Differential Revision: D26079717

fbshipit-source-id: 98d938bfced417e0b39a6dcddb241225ffb69c06

Author: chadaustin

build/fbcode_builder/getdeps/runcmd.py

@@ -51,7 +51,7 @@ def log_to_stdout(msg):
         sys.stdout.buffer.write(msg.encode(errors="surrogateescape"))
 
     if log_file is not None:
-        with open(log_file, "a", encoding="utf-8") as log:
+        with open(log_file, "a", encoding="utf-8", errors="surrogateescape") as log:
 
             def log_function(msg):
                 log.write(msg)