git-svn: use "no warnings 'once'" to disable false-positives

git-svn: use "no warnings 'once'" to disable false-positives

Some variables coming from the Subversion's Perl bindings are used
in our code only once, so the interpreter warns us about it.  These
warnings are false-positives, because the variables themselves are
initialized in the binding's guts, that are made by SWIG.

Credits to Sam Vilain for his note about "no warnings 'once'".

[ew: minor formatting change]

Signed-off-by: Eygene Ryabinkin <rea-git@codelabs.ru>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>