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 <redacted>
Acked-by: Eric Wong <redacted>
Signed-off-by: Shawn O. Pearce <redacted>