tg: reduce subshell creation phase II

tg: reduce subshell creation phase II

Continue eliminating excess subshell invocations.

In this phase replace all uses of ref_exists_rev and ref_exists_rev_short
with the new replacements v_ref_exists_rev and v_ref_exists_rev_short.

Use of rev_parse_tree is also replaced by v_rev_parse_tree but it's only
used internally by the branch_annihilated function which has been mostly
superseded by the much faster awk support.

With these changes the branch_empty and branch_contains functions can
potentially avoid spawning any subshells provided the cache is up-to-date.

This provides yet another nice minor speed boost to `tg summary`.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>