trydiff: inline sole addmodehdr() call
Now that there is only a single call to addmodehdr() left, and there
is other similar code (for new/deleted files) around that call site,
let's inline the function there. That also makes it clearer under what
circumstances the header is actually written (when modes differ).
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..