cmd/viewcore: fix commandPath recursion

commandPath recursively called itself with the same command when the
command had a parent. The recursion therefore never reached the root
command and eventually caused a stack overflow.

Recurse with c.Parent() so each call advances toward the root. Add
table-driven tests covering root, one-level, and two-level command paths.

Change-Id: I8d258a9f2d4d2c42c1cd66df0873fa99185a8aa9
Reviewed-on: https://go-review.googlesource.com/c/debug/+/811640
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>