The package "gnuit" (GNU Interactive Tools, a file browser/viewer and process viewer) was called "git" in Debian up until 2009-09-09, while git was called "git-core".
Therefore, a better graph to look at is:
hXXp://qa.debian.org/popcon-png.php?packages=git-core%20git&show_installed=on&want_legend=on&want_ticks=on&date_fmt=%25Y-%25m&beenhere=1 (can someone please fix this for me? I have not got enough reputation around here)
Which shows that the popularity did not rise dramatically (take the green line for the left part until they cross, then take the red line).