Git log should be incremental

Right now Qt Creator uses git log -n100 to obtain file history, and allows to change number from 100 to other value. On large repositories -n100 is too slow, and using small numbers like -n5 may become insufficient in case older entries are needed. To the contrary, git log in terminal is always blazing fast, because it loads only those entries which are visible on the screen, and loads older when user navigates down.

I think easiest solution is to use small number of log entries by default, and request new "page" when user reaches the bottom (applying same git log -n command to oldest hash from loaded part of log ~1)

More complex solution would be to use libgit2 and its git log iterators, but that would require major architectural changes (while providing smoother user experience)