tg: reduce subshell creation phase III

tg: reduce subshell creation phase III

More excess subshell invocation eliminations.

In this phase replace all uses of verify_topgit_branch with the
existing v_verify_topgit_branch and remove the verify_topgit_branch
wrapper function.

Although there's unlikely to be much of a noticeable speed-up in
this case, it cleans up the code and completes the transition that
was begun some time ago.  With the other recent changes, the
v_verify_topgit_branch function can now often do its work without
creating any new subshells (aka forking).

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