Pkg: Don't spawn git processes asynchronously

This change significantly reduces the system overhead by spawning many
git commands asynchronously. By running each git command sequentially,
this change also restores Pkg.update() functionality to Mac OS X
systems rendered unusable by JuliaLang#8601.

base/pkg/entry.jl

@@ -254,18 +254,18 @@ function update(branch::AbstractString)
  end
  avail = Read.available()
  # this has to happen before computing free/fixed
- @sync for pkg in filter!(Read.isinstalled,collect(keys(avail)))
- @async Cache.prefetch(pkg, Read.url(pkg), [a.sha1 for (v,a)=avail[pkg]])
+ for pkg in filter!(Read.isinstalled,collect(keys(avail)))
+ Cache.prefetch(pkg, Read.url(pkg), [a.sha1 for (v,a)=avail[pkg]])
  end
  instd = Read.installed(avail)
  free = Read.free(instd)
- @sync for (pkg,ver) in free
- @async Cache.prefetch(pkg, Read.url(pkg), [a.sha1 for (v,a)=avail[pkg]])
+ for (pkg,ver) in free
+ Cache.prefetch(pkg, Read.url(pkg), [a.sha1 for (v,a)=avail[pkg]])
  end
  fixed = Read.fixed(avail,instd)
- @sync for (pkg,ver) in fixed
+ for (pkg,ver) in fixed
  ispath(pkg,".git") || continue
- @async begin
+ begin
  if Git.attached(dir=pkg) && !Git.dirty(dir=pkg)
  info("Updating $pkg...")
  @recover begin