hook: add sign-off using "interpret-trailers"

The sample hook to prepare the commit message before
a commit allows users to opt-in to add the sign-off
to the commit message. The sign-off is added at a place
that isn't consistent with the "-s" option of "git commit".
Further, it could go out of view in certain cases.

Add the sign-off in a way similar to "-s" option of
"git commit" using git's interpret-trailers command.

It works well in all cases except when the user invokes
"git commit" without any arguments. In that case manually
add a new line after the first line to ensure it's consistent
with the output of "-s" option.

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--prepare-commit-msg.sample

@@ -32,4 +32,8 @@ SHA1=$3
 # esac
 
 # SOB=$(git var GIT_AUTHOR_IDENT | sed -n 's/^\(.*>\).*$/Signed-off-by: \1/p')
-# grep -qs "^$SOB" "$COMMIT_MSG_FILE" || echo "$SOB" >> "$COMMIT_MSG_FILE"
+# git interpret-trailers --in-place --trailer "$SOB" "$COMMIT_MSG_FILE"
+# if test -z "$COMMIT_SOURCE"
+# then
+#   @PERL_PATH@ -i.bak -pe 'print "\n" if !$first_line++' "$COMMIT_MSG_FILE"
+# fi