Fix default-directory issue with magit-grep.

As of now, magit-grep doesn't set the default-directory variable
correctly when creating the *Magit Grep* buffer.  This is especially
problematic if the user tries to use the file name / line number pair as
a link.
Merge pull request #699 from sanjoy/default-dir-in-magit-grep

Fix default-directory issue with magit-grep.