Make pin reftest regexp more robust

With the new `git -C` commands showing, the regexp could
potentially match on the path passed to the `-C` option.

Signed-off-by: Nathan Rebours <nathan.rebours@ocamlpro.com>

Author: NathanReb

tests/reftests/pin.test

@@ -240,7 +240,7 @@ The following actions will be performed:
 <><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
 -> installed nip-git2.ved
 Done.
-### opam pin add ./nip-path2 --kind git | sed-cmd git | "128.*" -> "128"
+### opam pin add ./nip-path2 --kind git | sed-cmd git | " 128.*" -> " 128"
 [ERROR] Command "git -C ${BASEDIR}/nip-path2 ls-files" failed:
 "git -C ${BASEDIR}/nip-path2 ls-files" exited with code 128
 # Return code 99 #