Fix file paths in projectile-replace* with prefix arg

[bcc32]

When prefix arg is supplied and a directory is selected, the directory
passed to projectile-dir-files is not necessarily the project root, so
using projectile-expand-root yields incorrect behavior.

For example, in a project rooted at ~/project with files:

~/project/
~/project/Makefile
~/project/foo/bar.c

With C-u M-x projectile-replace-regexp, selecting directory ~/project/foo/, projectile incorrectly attempts to read file ~/project/bar.c, resulting in an error.

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Thanks!

When running eldev test, I see that two tests fail, but they also fail in master, so I don't think it has anything to do with this PR.

[bbatsov]

The changes look good to me. Thanks!