completion: fix incorrect bash/zsh string equality check

completion: fix incorrect bash/zsh string equality check

In the basic `[`/`test` command, the string equality operator is a
single `=`. The `==` operator is only available in `[[`, which is a
bash-ism also supported by zsh.

This mix-up was causing the following completion error in zsh:
> __git_ls_files_helper:7: = not found

(That message refers to the extraneous symbol in `==` ← `=`).

This updates that comparison to use a single `=` inside the
basic `[ … ]` test conditional.

Although this fix is inconsistent with the other comparisons in this
file, which use `[[ … == … ]]`, and the two expressions are functionally
identical in this context, that approach was rejected due to a
preference for `[`.

Signed-off-by: Robert Estelle <redacted>
Signed-off-by: Junio C Hamano <redacted>