prepare-commit-msg: add 50 column marker

A common convention suggests a 50 column commit subject line (the first
line of the commit message).  Add a 50 column marker to the commit
message template

Reviewed by:	lwhsu
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45538

tools/tools/git/hooks/prepare-commit-msg

@@ -38,7 +38,7 @@ outfile=$(mktemp /tmp/freebsd-git-commit.XXXXXXXX)
 
 cat >$outfile <<EOF
 $(awk '1;/^#$/{exit}' $1)
-#                                                         72 columns --|
+#                                   50 columns --|        72 columns --|
 #
 # Uncomment and complete these metadata fields, as appropriate:
 #