hook: name the positional variables

It's always nice to have named variables instead of
positional variables as they communicate their purpose
well.

Appropriately name the positional variables of the hook
to make it easier to see what's going on.

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

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

@@ -19,14 +19,17 @@
 # The second example adds a Signed-off-by line to the message, that can
 # still be edited.  This is rarely a good idea.
 
+COMMIT_MSG_FILE=$1
+COMMIT_SOURCE=$2
+SHA1=$3
 
-# case "$2,$3" in
+# case "$COMMIT_SOURCE,$SHA1" in
 #  ,|template,)
 #    @PERL_PATH@ -i.bak -pe '
 #       print "\n" . `git diff --cached --name-status -r`
-# 	 if /^#/ && $first++ == 0' "$1" ;;
+# 	 if /^#/ && $first++ == 0' "$COMMIT_MSG_FILE" ;;
 #  *) ;;
 # esac
 
 # SOB=$(git var GIT_AUTHOR_IDENT | sed -n 's/^\(.*>\).*$/Signed-off-by: \1/p')
-# grep -qs "^$SOB" "$1" || echo "$SOB" >> "$1"
+# grep -qs "^$SOB" "$COMMIT_MSG_FILE" || echo "$SOB" >> "$COMMIT_MSG_FILE"