patch: make _addmodehdr a function under trydiff
addmodehdr is a header helper, same as diffline, so it doesn't
need to be a top-level function and can be nested under trydiff.
In upcoming patches we will generalize this approach for
all headers.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda
$ cd ..