annotate: build format string separately from annotation data
This prepares for porting to generic templater API.
Note that we cannot use '%*s' to pad white spaces because it doesn't take
into account character widths, as described in
4f5a6df2af92.
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
revision 0
branch default
phase draft
adda
$ cd ..