git_config: add SetBoolean helper

A little sugar simplifies the code a bit.

Change-Id: Ie2b8a965faa9f9ca05c7be479d03e8e073cd816d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296522
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/git_config.py b/git_config.py
index 2fa43a1..282c080 100644
--- a/git_config.py
+++ b/git_config.py
@@ -161,6 +161,12 @@
       return False
     return None
 
+  def SetBoolean(self, name, value):
+    """Set the truthy value for a key."""
+    if value is not None:
+      value = 'true' if value else 'false'
+    self.SetString(name, value)
+
   def GetString(self, name, all_keys=False):
     """Get the first value for a key, or None if it is not defined.