bash_completion: determine root executable in case of alias

This solves an issue for users that have "alias hg='hg --some_opts'" where they
would see wrong entries in the completion output (such as color codes).

contrib/bash_completion

@@ -130,12 +130,19 b' shopt -s extglob'
 
 _hg()
 {
-    local cur prev cmd cmd_index opts i
+    local cur prev cmd cmd_index opts i aliashg
     # global options that receive an argument
     local global_args='--cwd|-R|--repository'
     local hg="$1"
     local canonical=0
 
+    aliashg=$(alias $hg 2>/dev/null)
+    if [[ -n "$aliashg" ]]; then
+      aliashg=${aliashg#"alias $hg='"}
+      aliashg=${aliashg%"'"}
+      hg=$aliashg
+    fi
+
     COMPREPLY=()
     cur="$2"
     prev="$3"