Add ability to disable git submodules

[toolmantim]

Allows using the agent in environments that don't have Perl, which the git submodule commands rely upon.

Related to #52

[blueimp]

Thanks for the quick implementation, @toolmantim!

If you change the two lines with

if [[ ! -z "${BUILDKITE_DISABLE_GIT_SUBMODULES:-}" ]]; then

to

if [[ -z "${BUILDKITE_DISABLE_GIT_SUBMODULES:-}" ]]; then

it will work fine. :)

From the naming of the variable, I guess you wanna check if BUILDKITE_DISABLE_GIT_SUBMODULES is empty, rather than not empty before executing the git submodule calls.

[toolmantim]

Whoops, fixed. Give that a go and happy to merge into master if it works 👍

[blueimp]

Tested and works! 👍

[toolmantim]

@blueimp awesome! Thanks for testing. This will go out in the next agent release, which should be within the next week.

[blueimp]

Thanks! :)