Work around MinGW mangling of "host:/path"

Work around MinGW mangling of "host:/path"

The common way to specify an ssh remote is to say "host:/path",
but MinGW decides for us that this is probably a path list, and
path lists are separated by a semicolon on Windows.  So before
passing this to git-peek-remote.exe, it transforms that to
"host;C:/msysGit/path".  Avoid that by expanding it to
"ssh://host/path", but take extra care not to convert absolute
paths to that syntax!

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>