setup: avoid double slashes when looking for HEAD

Andrew Baumann reported that when called outside of any Git worktree, `git rev-parse --is-inside-work-tree` eventually tries to access `//HEAD`, i.e. any `HEAD` file in the root directory, but with a double slash. This double slash is not only unintentional, but is allowed by the POSIX standard to have a special meaning. And most notably on Windows, it does, where it refers to a UNC path of the form `//server/share/`. As a consequence, afore-mentioned `rev-parse` call not only looks for the wrong thing, but it also causes serious delays, as Windows will try to access a server called `HEAD`. Let's simply avoid the unintended double slash. Signed-off-by: Jeff King <peff@peff.net> Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>