git-svn: ignore leading blank lines in svn:ignore

git-svn: ignore leading blank lines in svn:ignore

Subversion ignores all blank lines in svn:ignore properties.  The old
git-svn code ignored blank lines everywhere except for the first line
of the svn:ignore property.  This patch makes the "git svn
show-ignore" and "git svn create-ignore" commands ignore leading blank
lines, too.

Also include leading blank lines in the test suite.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>