t7900 failures when $HOME is symlinked

Hi Stolee,

I noticed two failures, t7900.32 and t7900.36, on a system where
$HOME is symlinked, i.e.

     $ cd $HOME && pwd
     /home/me
     $ pwd -P
     /some/other/path/me

These two tests use 'pfx = $(cd $HOME && pwd)', so $pfx is '/home/me',
but the actual path that gets written by Git is canonicalized, i.e.
'/some/other/path/me'. I think a simple fix would be to use 'pwd -P'
instead, which fixes it for me.

Cheers,

Philippe.