git-completion.bash: rename to $__git_cmd_idx
In
e94fb44042 (git-completion.bash: pass $__git_subcommand_idx from
__git_main(), 2021-03-24), the $__git_subcommand_idx variable was
introduced. Naming it after the index of the subcommand is needlessly
confusing as, when this variable is used, it is in the completion
functions for commands (e.g. _git_remote()) where for `git remote add`,
the `remote` is referred to as the command and `add` is referred to as
the subcommand.
Rename this variable so that it's obvious it's about git commands. While
we're at it, shorten up its name so that it's still readable without
being a handful to type.
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>