Set default to master.

This seems to change to main according to
https://lore.kernel.org/git/pull.762.git.1605221038.gitgitgadget@gmail.com/.
Pre-emptive change (since I don't know when I'll be affected by it).

.config/git/config

@@ -53,6 +53,8 @@
     smudge = ANSIBLE_VERBOSITY=0 ansible-vault decrypt
 [grep]
     linenumber = true
+[init]
+    defaultBranch = master
 [merge]
     tool = meld
     ff = false