*.bash: always use the same string equality operator

*.bash: always use the same string equality operator

POSIX Shell only supports = to compare variables inside '[' tests. But
this is Bash, where == is an alias for =. In practice they're the same,
but the current form is inconsisnent and breaks POSIX for no good
reason.

Change-Id: I38fa7a5a90658dc51acc2acd143049e510424ed8
Reviewed-on: https://go-review.googlesource.com/38031
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>