`git-extras update` disregards aliases

If I install git-extras manually, I get prompts for conflicting aliases, and can skip those.

~/local/opp/git/git-extras
[~/local/opp/git/git-extras on ⇄ relocatable-etc]
$ make PREFIX=/tmp/gitex SYSCONFPREFIX=/tmp/gitex install
... installing bins to /tmp/gitex/bin
... installing man pages to /tmp/gitex/share/man/man1
... installing git-archive-file
... installing git-authors
... installing git-back
git-bug conflicts with an alias, still install it and disable the alias? [y/n]n
... installing git-changelog

With git-extras update, it doesn't issue the prompt, and just clobbers all the aliases with commands.

$ git config --get-regexp 'alias.*'
alias.bug git checkout
[~]
$ git-extras update
...
... installing git-back
... installing git-bug
... installing git-changelog
... installing git-chore
... installing git-commits-since

I suspect it's because the curl -s | bash form that git-extras update uses from #381 is incompatible with the prompting the Makefile does: it can't read user response from stdin because the script it's executing is on stdin instead.

Start with bin/git-extras, especially the update flow linked in the issue, and compare it with the prompting behavior in the Makefile. Reproduce the alias conflict using the shown git config and git-extras update commands; done means update handles the existing alias instead of silently clobbering it.