git-submodule-bump failed to extract commit history

Trying to use the git-submodule-bump GDP helper script to create a g-d-p submodule update commit I found it reported two warnings/errors and failed to add the change history to the commit message as expected.

Steps to reproduce
1. Checkout GDP Master sha b21192b22
2. Update the renesas-rcar-gen3 submodule to sha aa6a28bb7 (do not add the change to the git index/cache - the script assumes it has not been added)
3. run git-submodule-bmp to create submodule update commit, e.g:
./git-submodule-bump "Rebase on Yocto BSP v2.19"
4. Script will report two warnings and create a commit without the expected summary of changes.

Example session:

steve@be1yocto:~/genivi/genivi-12/m3sk/genivi-dev-platform$git checkout -b test origin/master
M renesas-rcar-gen3
Branch test set up to track remote branch master from origin.
Switched to a new branch 'test'
steve@be1yocto:~/genivi/genivi-12/m3sk/genivi-dev-platform$ git status
On branch test
Your branch is up-to-date with 'origin/master'.

Changes not staged for commit:
(use "git add <file>..." to update what will be committed)
(use "git checkout – <file>..." to discard changes in working directory)
(commit or discard the untracked or modified content in submodules)

modified: renesas-rcar-gen3 (new commits, untracked content)

[snip]
steve@be1yocto:~/genivi/genivi-12/m3sk/genivi-dev-platform$ ./git-submodule-bump "Rebase on Yocto BSP v2.19"
./git-submodule-bump: 29: ./git-submodule-bump: MSG+=$\n\n: not found
./git-submodule-bump: 30: ./git-submodule-bump: MSG+=aa6a28b: not found
[test 01cc26b] renesas-rcar-gen3: Rebase on Yocto BSP v2.19
1 file changed, 1 insertion, 1 deletion
steve@be1yocto:~/genivi/genivi-12/m3sk/genivi-dev-platform$ git log
commit 01cc26b379d09eee9d50c4ad814a97ad890ae45e
Author: Stephen Lawrence <stephen.lawrence@renesas.com>
Date: Thu Jun 8 20:11:48 2017 +0100

renesas-rcar-gen3: Rebase on Yocto BSP v2.19

Signed-off-by: Stephen Lawrence <stephen.lawrence@renesas.com>

commit b21192b22d9447d2e80bfdb1f262f0b763dee554
Author: Gunnar Andersson <gandersson@genivi.org>
Date: Sat May 27 20:07:35 2017 +0200

ci-build.sh: Update submodules before printing them