text/template: avoid allocating a new common in copy

Template.New calls t.init, which allocates several items that
are immediately rewritten by copy, so avoid the call to New

Change-Id: I16c7cb001bbcd14cf547c1a2db2734a2f8214e7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/182757


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>