change directory to the base of your project by looking for '.git'

base.sh

# change directory to the base of your project

# Your project base is the directory that contains
# either _darcs or .git

# to use as a command add to .zshrc or .bashrc
# alias base='source <path/to/base.sh>

CUR=$(pwd);
while [ ! "$CUR" = "/" ]
do
    CUR=$(dirname "$CUR");
    ([ -d "$CUR/.git" ] || [ -d "$CUR/_darcs" ]) && echo "Changing directory to $CUR" && cd "$CUR" && break  
done