style: Don't include diff context in git style hook

style: Don't include diff context in git style hook

The git style hook currently includes a few lines of diff context when
determining changed regions. This is undesirable as this triggers
false positives when modifying existing files with a lot of style
violations. This change sets the amount of context to 0, which is the
default value when requesting staged regions from the git helper.

Change-Id: Ibe03123e329ea0241281e104183a68d6c495b190
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nathanael Premillieu <nathanael.premillieu@arm.com>