completion: add a debuglabelcomplete command When completing a "label" (a symbolic name for a commit), the bash_completion script currently has to invoke hg three times. For a large repository, the cost of starting up and loading all the necessary context over and over is very high. For instance, in mozilla-central: time (export HGPLAIN=1; hg tags -q; hg bookmarks -q; hg branches) >/dev/null 0.446 sec Compare with the debuglabelcomplete command that this commit adds: time hg debuglabelcomplete >/dev/null 0.148 sec This greatly helps responsiveness.