BENCH: Default to building HEAD instead of master

asv executes the benchmarks of the current branch building the code
specified in "branches". Previously this was "master" instead of "HEAD"
(the code currently worked on). This PR changes "master" -> "HEAD" as it
seems more likely for a user to be generating benchmarks for the code
they are currently working on.

Author: Kai-Striega

benchmarks/asv.conf.json

@@ -15,7 +15,7 @@
 
     // List of branches to benchmark. If not provided, defaults to "master"
     // (for git) or "tip" (for mercurial).
-    "branches": ["master"],
+    "branches": ["HEAD"],
 
     // The DVCS being used.  If not set, it will be automatically
     // determined from "repo" by looking at the protocol in the URL