always write log files as UTF-8 to avoid cp1252 encoding errors

Summary:
The log file should be opened with a defined encoding rather than
whatever the platform default is. On Windows, that might be cp1252,
which throws an encoding error when the text is unicode.

Reviewed By: genevievehelsel

Differential Revision: D26025779

fbshipit-source-id: 0102b2555e91812dfe94a7c332eb896ca2c9ef7e

build/fbcode_builder/getdeps/runcmd.py

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