git-svn: Use GIT_SSH setting if SVN_SSH is not set

git-svn: Use GIT_SSH setting if SVN_SSH is not set

If SVN_SSH is defined, it will be used. Else value in
GIT_SSH is copied to SVN_SSH & then, only on Windows,
the \s are escaped.

On Windows, the shell-variables must be set as follows
    GIT_SSH="C:\Program Files\PuTTY\plink.exe"
    SVN_SSH="C:\\Program Files\\PuTTY\\plink.exe"

See http://code.google.com/p/msysgit/issues/detail?id=305

[ew: fixed indentation to use tabs]

Acked-by: Eric Wong <redacted>
Signed-off-by: Karthik Rajagopalan <redacted>