This makes the PBGitRevisionCell a bit nicer by retrieving all values
from the PBGitCommit object itself, and using another NSTextFieldCell to
draw the text.
This mean that PBGitGrapher now stores its information in the PBGitCommit's,
rather than in a custom grapher array. Also, because we don't need the
grapher to display refs anymore, the ref labels are also displayed when
using path limiting (for example, 'gitx -- Makefile').
This uses more Cocoa classes to draw the lines, making it easier to
understand and hopefully maintain.
Furthermore, we use less memory now, which is nice, but all the
dynamic arrays probably mean more CPU usage.
Benjamin Kramer