method 1
if you don't care about security, embed password in URL string
http://user:password@abcdef.com/abc/def.git
method 2
use a git credential helper to store username and password (more secure)
msysgit 1.8.1 or later: set config to enable the built-in git-credential-wincred [credential] helper = wincred
or install git-credential-winstore for older msysgit