view: add more view column types for the branch and tree view
view: move io_memchr() to the io module

Make io_memchr() available as part of the I/O API. This also fixes
io_get() to ensure that the I/O buffer members are set properly for the
final read compared to the previous reads.
view: rewrite branch_read to read commit info in one shot
view: add draw infrastructure for column based views
view: rename view columns title to commit title
view: use view columns infra for the main and stash views
view: add line-number column type to allow natural sorting order