Fix :Glog

Closes #545.
Fix instaweb support

Closes #571.
Make configured_tree a caching global function

This implements the changes suggested in tpope/fugitive#415.
s:repo_configured_tree is now a global, s:configured_tree() that caches
the bidirectional relation between the worktree and the git_dir.
extract_git_dir() now uses that relation to check whether the
directories it scans are valid worktrees known by the repo at $GIT_DIR.
Use `<nomodeline>` with Fugitive autocmds, and un-silent them

Closes #580.
Merge remote-tracking branch 'upstream/master'