Fix a bug in strip_path when prefix is a symlink
Fix a bug in strip_path when prefix is a symlink

helpers.realpath() wasn't the right choice for path normalization.
The prefix itself can be a symlink, too.  Now we use os.path.realpath(),
which also follows symlinks for the last element.

Signed-off-by: Zoran Zaric <d7dacae2c968388960bf8970080a980ed5c5dcb7@zoranzaric.de>

mhoeher/bup,tjanez/bup,gevaerts/bup,jbaber/bup,pombredanne/bup,mhoeher/bup,jbaber/bup,pombredanne/bup,pombredanne/bup,apenwarr/bup,apenwarr/bup,gevaerts/bup,mhoeher/bup,tjanez/bup,apenwarr/bup,tjanez/bup,ToxicFrog/bup,mhoeher/bup,jbaber/bup,gevaerts/bup,jbaber/bup,gevaerts/bup,apenwarr/bup,tjanez/bup,ToxicFrog/bup,jbaber/bup,ToxicFrog/bup,ToxicFrog/bup,pombredanne/bup,gevaerts/bup,mhoeher/bup,pombredanne/bup,mhoeher/bup,ToxicFrog/bup,tjanez/bup,jbaber/bup,pombredanne/bup,tjanez/bup