format-patch for a single file
If you give git format-patch a single revision, it will produce patches for each commit since that revision. If you see no output from that command, then I suspect that there were no changes to that file between origin/master and your current HEAD. As an alternative, you can provide a revision range (e.g. origin/master~3..origin/master) which covers the changes introduced to that file. Or, if the changes you want to produce a patch for are just contained in the single commit at the tip of origin/master, you can use the -1 parameter, as in:
git format-patch -1 origin/master test/test_module.c
You can use following syntax for creating patch for single file:
git format-patch [commit_hash] [file]