purge: add a --confirm option
The options provide a prompt to the user before permanent deletion are made.
The prompt is currently not aware of directory deletion. I'll fix this in the
next changesets.
Differential Revision: https://phab.mercurial-scm.org/D9818
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..