bash_completion: match more narrowly

This greatly helps completion performance for most commands that deal
with files.

In a working dir with 150,000 files, where we want to complete the name
of a modified file under a path beginning with "a", from the root of
the working dir:

  (old) hg status -nm .            1.7 sec
  (new) hg status -nm "glob:a**"   0.3

Even "hg add" becomes a little faster, in spite of being the worst
case (matching untracked files).

contrib/bash_completion

@@ -82,7 +82,7 b' shopt -s extglob'
 
 _hg_status()
 {
-    local files="$(_hg_cmd status -n$1 .)"
+    local files="$(_hg_cmd status -n$1 "glob:$cur**")"
     local IFS=$'\n'
     compopt -o filenames 2>/dev/null
     COMPREPLY=(${COMPREPLY[@]:-} $(compgen -W '$files' -- "$cur"))