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 <robertestelle@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>