expand_user_path: expand ~ to $HOME, not to the actual homedir.

expand_user_path: expand ~ to $HOME, not to the actual homedir.

In 395de250d (Expand ~ and ~user in core.excludesfile, commit.template),
we introduced the mechanism. But expanding ~ using getpw is not what
people overriding $HOME would usually expect. In particular, git looks
for the user's .gitconfig using $HOME, so it's better to be consistent.

Signed-off-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>