bash_completion: do not use aliased hg if it sources a script (issue6308)

I have an alias that sources a script around hg. Mercurial's bash_completion
script tries to use this as its main hg binary. But sourcing a wrapper breaks
Bash's completion. So this patch disables using the alias as the hg binary if
it starts with "source ".

Alias resolution was introduced in rev 191ab08e7099 for users with
"alias hg='hg --some_opts'".

See https://www.mercurial-scm.org/repo/hg/rev/191ab08e7099

contrib/bash_completion

@@ -160,8 +160,11 b' shopt -s extglob'
     if [[ -n "$aliashg" ]]; then
       aliashg=${aliashg#"alias $hg='"}
       aliashg=${aliashg%"'"}
+      # `source`d aliases break completion, so ignore them
+      if [[ "${aliashg:0:7}" != "source " ]]; then
       hg=$aliashg
     fi
+    fi
 
     COMPREPLY=()
     cur="$2"