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>

contrib/completion/git-completion.bash

@@ -515,7 +515,7 @@ __gitcomp_file ()
 # argument, and using the options specified in the second argument.
 __git_ls_files_helper ()
 {
-	if [ "$2" == "--committable" ]; then
+	if [ "$2" = "--committable" ]; then
 		__git -C "$1" -c core.quotePath=false diff-index \
 			--name-only --relative HEAD -- "${3//\\/\\\\}*"
 	else