[arcconfig] Default base to previous revision

When you run `arc diff`, arc defaults to uploading all the changes you
have against the upstream branch into a single patch. This is almost
never what you want for stacked commits (patch series); you only want to
submit the changes done by the current patch. It's also come up as a
point of confusion in the Phabricator vs. GitHub PRs discussion, for
example. Configure arc to only upload your current patch by default,
which I think is a much more suitable default for LLVM developers.

Differential Revision: https://reviews.llvm.org/D74990

.arcconfig

@@ -1,5 +1,6 @@
 {
   "phabricator.uri" : "https://reviews.llvm.org/",
   "repository.callsign" : "G",
-  "conduit_uri" : "https://reviews.llvm.org/"
+  "conduit_uri" : "https://reviews.llvm.org/",
+  "base": "git:HEAD^"
 }