Windows: Fix bug that caused git-annex sync to fail due to missing environment variable.

I think that the problem was caused by windows not having a concept of an
env var that is set, but to the empty string. So, GIT_ANNEX_SSHOPTION
got set to "" and was not seen as set at all.

Easy fix, which also makes git-annex sync a little faster is to not set
GIT_SSH, when GIT_ANNEX_SSHOPTION has no options. Might as well let git use
ssh per usual in this case, no need to run git-annex as the proxy ssh
command..

doc/bugs/git-annex_sync_under_windows_fails_by_using_itself_as_the_ssh_command.mdwn

@@ -213,3 +213,5 @@ C:\data_organization\data>
 
 
 """]]
+
+> [[fixed|done]] I think. --[[Joey]]