commit-binaries: fix --dry-run usage

When passing the--dry-run option, commit-binaries exits with a error code = 1 because:

• git add --dry-run
• git commit --dry-run # this returns 1 error code

We must have something to stage, even if git commit uses --dry-run.

Stage all files and unstage them afterwards in case of dry-run.

Signed-off-by: Mattijs Korpershoek mkorpershoek@baylibre.com