Have 'nightly' svn-commit performance data upon -performance runs.

[reviewed by Lydia]

Adding the following actions to 'nightly ... -performance',
to be performed in the NightlyPerformance/$host directory
(where the *.dat files from nightly performance runs reside):

* before running the tests:
   svn update

* after running the tests:
   svn update
   svn add *.dat  # failures are ignored
   svn commit

Failure of any of the above, i.e. error exit code, will not affect
anything downstream. The only effect will be that the summary email
will go to the "everyone" list, just like when having new or resolved
failures or new passing futures.

The only reflection of the success of the svn commands in the summary
email is the error count, from 0 (all succeeded) to 3 (all failed,
except 'svn add' might have succeeded).

'svn add' will most likely fail because there are *.dat files that
are already under svn. For now I do not try to analyze if there is
anything worth sending email about, w.r.t. 'svn add'.

I added the function showsystem() that prints the command before
executing it. Alternatively I could add another argument to mysystem(),
but then at call sites I'd have to pass 4 extra arguments, compared
to just the command to pass to showsystem().

Running 'svn update' twice (before and after testing) is intended to
reduce the possibility of conflicts in 'svn update' or 'svn add'.
While they (the conflicts) can still happen, it is not very likely, so
for now I do not have any special handling for them (the conflicts).


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@22143 3a8e244f-b0f2-452b-bcba-4c88e055c3ca