contrib/completion: fix zsh completion regression from 59d85a2a05

contrib/completion: fix zsh completion regression from 59d85a2a05

A recent change to make git-completion.bash use $__git_cmd_idx
in more places broke a number of completions on zsh because it
modified __git_main but did not update __git_zsh_main.

Notably, completions for "add", "branch", "mv" and "push" were
broken as a result of this change.

In addition to the undefined variable usage, "git mv <tab>" also
prints the following error:

	__git_count_arguments:7: bad math expression:
	operand expected at `"1"'

	_git_mv:[:7: unknown condition: -gt

Remove the quotes around $__git_cmd_idx in __git_count_arguments
and set __git_cmd_idx=1 early in __git_zsh_main to fix the
regressions from 59d85a2a05.

This was tested on zsh 5.7.1 (x86_64-apple-darwin19.0).

Suggested-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Acked-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>