Remove references to acrepo.h
BUG: Fix boot times issues by removing const

By removing the const keyword it has allowed the commit object to lookup
the tree of the commit when it is needed and not at creation time. This
makes the startup time when loading the libgit2 repo decrease from 1:36
to 4 seconds.
BUG: Stopped checking if there is changes in the working directory

This was occurring on every call to the paint routine which was horrible!

For now a variable will stored by the mainwindowreview object and updated
when appropriate from mainwindow.
BUG: Check index limits before blindly using the index

When holding down the down button to navigate through the commits it would
crash due to going out of bounds when trying to go beyond the first commit
in the repository.