[PATCH] tests: do not use system hg if it does not have "files" command

[Jun Wu]

Excerpts from Adam Simpkins's message of 2017-06-28 12:54:18 -0700:
> Looks good to me, but this will need to be updated to the
> non-bash-specific function syntax (e.g., 'syshg () {'),
> as Yuya pointed out on my earlier diff.
> 
> If you wanted to be pedantic you could also replace "true" with ":"
> here; ":" is guaranteed to be a shell builtin; "true" may require
> executing a separate process on some systems.  This probably doesn't
> really matter though.
 
Good advice. I have been wondering the usage of ":" except for fork bomb for
a while. I'll also fix English grammar in the comment.