[build] Search for the git binary in the default msysgit install dir

The Git for Windows installer doesn't add the Git binaries to the path
by default. (Due to risk of conflicts with built-in windows commands.)
Unless you have configured your system specially this causes the
scm_rev_gen.js script to fail to find Git.  Added more paths to the
script so that it searches in the default msysgit installation
directory, eliminating the need to set the PATH for most environments.