git-svn.txt: replace .git with $GIT_DIR

git-svn.txt: replace .git with $GIT_DIR

As $GIT_DIR may not equal '.git', it's usually more generally correct to
refer to files in $GIT_DIR rather than in .git .

This will also allow me to link some of the occurrences of '.git' in
git-svn.txt to a new reference target inside this file in an upcoming
commit, because in AsciiDoc definitions apparently can't start with
a '.' character.

Signed-off-by: Keshav Kini <keshav.kini@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>