.hooks/prepare-commit-msg: Cleanup.

Instead of writing everything in the new commit template all at once in
a large here document, do it cleanly one command at a time.

.hooks/prepare-commit-msg

@@ -26,6 +26,7 @@ merge)
 esac
 
 outfile=$(mktemp /tmp/freebsd-git-commit.XXXXXXXX)
+exec 3> "$outfile"
 
 # Create a commit message template from three parts:
 #
@@ -38,8 +39,9 @@ outfile=$(mktemp /tmp/freebsd-git-commit.XXXXXXXX)
 #    line to the end of the file) which lists files staged for commit, files
 #    not staged, and untracked files.
 
-cat >"$outfile" <<EOF
-$(awk '1;/^#$/{exit}' "$1")
+awk '1;/^#$/{exit}' "$1" >&3
+
+cat >&3 <<EOF
 # category/port: Subject goes here, max 50 cols -|
 # <then a blank line>
 #                                                         72 columns --|
@@ -65,7 +67,10 @@ $(awk '1;/^#$/{exit}' "$1")
 #
 # "Pull Request" and "Differential Revision" require the *full* GitHub or
 # Phabricator URL.
-$(awk '/^#$/,EOF' "$1")
 EOF
 
+awk '/^#$/,EOF' "$1" >&3
+
+exec 3>&-
+
 mv "$outfile" "$1"