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>

t/t9001-send-email.sh

@@ -1537,7 +1537,7 @@ test_expect_success $PREREQ 'sendemail.aliasfiletype=mailrc' '
 
 test_expect_success $PREREQ 'sendemail.aliasfile=~/.mailrc' '
 	clean_fake_sendmail &&
-	echo "alias sbd  someone@example.org" >~/.mailrc &&
+	echo "alias sbd  someone@example.org" >"$HOME/.mailrc" &&
 	git config --replace-all sendemail.aliasesfile "~/.mailrc" &&
 	git config sendemail.aliasfiletype mailrc &&
 	git send-email \