t9001: write $HOME/, not ~/, to help shells without tilde expansion

Even though it is in POSIX, we do not have to use it, only to hurt shells that may lack the support. The .mailrc test tries to define an alias in .mailrc in the home directory by shell redirection, and then tries to see ~/.mailrc in config is tilde-expanded by Git without help from shell. So the creation should become $HOME/ to be portable for shells that may lack tilde expansion but the reference should be done as "~/.mailrc". Signed-off-by: Junio C Hamano <gitster@pobox.com>