sharness: Use sed in a cross-platform safe way

OS X sed is documented as "-i SUFFIX", GNU sed as "-iSUFFIX". The one
consistent case seems to be "-iSUFFIX", where suffix cannot empty (or
OS X will parse the next argument as the suffix).

This used to leave around files named `refsout=` on Linux, and was
just confusing.

License: MIT
Signed-off-by: Jeromy <jeromyj@gmail.com>

test/sharness/t0080-repo.sh

@@ -138,7 +138,7 @@ test_expect_success "adding multiblock random file succeeds" '
 test_expect_success "'ipfs pin ls --type=indirect' is correct" '
 	ipfs refs "$MBLOCKHASH" >refsout &&
 	ipfs refs -r "$HASH_WELCOME_DOCS" >>refsout &&
-	sed -i="" "s/\(.*\)/\1 indirect/g" refsout &&
+	sed -i"~" "s/\(.*\)/\1 indirect/g" refsout &&
 	ipfs pin ls --type=indirect >indirectpins &&
 	test_sort_cmp refsout indirectpins
 '
@@ -166,7 +166,7 @@ test_expect_success "'ipfs pin ls --type=recursive' is correct" '
 	echo "$HASH_WELCOME_DOCS" >>rp_expected &&
 	echo "$EMPTY_DIR" >>rp_expected &&
 	ipfs refs -r "$HASH_WELCOME_DOCS" >>rp_expected &&
-	sed -i="" "s/\(.*\)/\1 recursive/g" rp_expected &&
+	sed -i"~" "s/\(.*\)/\1 recursive/g" rp_expected &&
 	ipfs pin ls --type=recursive >rp_actual &&
 	test_sort_cmp rp_expected rp_actual
 '