test case for regression caused by git-svn empty symlink fix

test case for regression caused by git-svn empty symlink fix

Commit dbc6c74d0858d77e61e092a48d467e725211f8e9 "git-svn: handle empty
files marked as symlinks in SVN" caused a regression in an unusual case
where a branch has been created in SVN, later deleted and then created
again from another branch point and the original branch point had empty
files not in the new branch. In some cases git svn fetch will then fail
while trying to fetch the empty file from the wrong SVN revision.

This adds a test case that reproduces the issue.

[ew: added additional test to ensure file was created correctly
     made test file executable ]

Signed-off-by: Anton Gyllenberg <anton@iki.fi>
Acked-by: Eric Wong <normalperson@yhbt.net>