git-svn: fix handling of filenames with embedded '@'

git-svn: fix handling of filenames with embedded '@'

svn has trouble parsing files with embedded '@' characters.  For
example,

  svn propget svn:keywords foo@bar.c
  svn: Syntax error parsing revision 'bar.c'

I asked about this on #svn and the workaround suggested was to append
an explicit revision specifier:

  svn propget svn:keywords foo@bar.c@BASE

This patch appends '@BASE' to the filename in all calls to 'svn
propget'.

Patch originally by Seth Falcon <sethfalcon@gmail.com>
Seth: signoff?

[ew: Made to work with older svn that don't support peg revisions]

Signed-off-by: Eric Wong <normalperson@yhbt.net>