LU-1595 build: improve prepare-commit-msg git hook

LU-1595 build: improve prepare-commit-msg git hook

Improve the Git prepare-commit-msg hook to still add the default
comment for Signed-off-by: and the Vim modeline, even if checkpatch.pl
isn't available.

Add a few more comments to the script.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I253931c34ee64afa2d9cf160e675c779b32e9359
Reviewed-on: http://review.whamcloud.com/13427
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>