add-header: do not add a header if the file starts with // already
add-header: do not add a header if the file starts with // already

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

This used to be revision r4562.

TehSAUCE/imagej,TehSAUCE/imagej,biovoxxel/imagej,biovoxxel/imagej,TehSAUCE/imagej,biovoxxel/imagej