gifn: add a shellcheck config file

Change-Id: I78e868cf81032e39da07dda4b3b27787fc5ba1ed
diff --git a/tools/git-if-needed/shellcheckrc b/tools/git-if-needed/shellcheckrc
new file mode 100644
index 0000000..2a4d199
--- /dev/null
+++ b/tools/git-if-needed/shellcheckrc
@@ -0,0 +1,3 @@
+shell=dash
+enable=all
+disable=SC2250