Fix bash_completion on Solaris
TK Soh noticed that the awk that ships with Solaris doesn't allow you
to define new functions. According to
http://www.shelldorado.com/articles/awkcompat.html
it looks like it also doesn't have some other stuff that we were using,
like deletion of an array element and gsub.
Rewrite the parsing of hg --debug help to work around that. This
version doesn't filter aliases that are simple abbreviations for debug
commands (not a big problem, since there are none right now).
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