Fix slowness when searching for networked git repos under Cygwin.

The algorithm in fugitive#extract_git_dir() is to move upwards in the
file system hierarchy until a sub-directory called .git is found. When
accessing a file on a network share from a Cygwin Vim and the file is not
within a git repo, this eventually causes a check for the existence of
//serverName/.git and //.git. Such checks are extremely slow so let's
avoid them.