Fix "Argument list too long"
Fix "Argument list too long"

While using rbenv and commiting a submodule commit with a large number
of commits causes an "Argument list too long".

Instead of passing a large variable, pass in the ref and shell out to
git to append the submodule changes.

Change-Id: Iaa4981c3c1998174430e08a2c06fba9c4f70eee4
Reviewed-on: https://gerrit.causes.com/26338
Tested-by: jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@causes.com>
Reviewed-by: Greg Hurrell <79e2475f81a6317276bf7cbb3958b20d289b78df@causes.com>
Reviewed-by: Lann Martin <564fa7a717c51b612962ea128f146981a0e99d90@causes.com>
Reviewed-by: John Skopis <086dbf0af61a04c5d5edda17e6cc474e61e60f4a@causes.com>
Tested-by: John Skopis <086dbf0af61a04c5d5edda17e6cc474e61e60f4a@causes.com>

TheSooth/overcommit,oxocode/overcommit,blech75/overcommit,TheSooth/overcommit,oxocode/overcommit,blech75/overcommit,gnagel/overcommit,gnagel/overcommit