smudge filter means that the clean filter _must_ accept its own output
without modifying it.
+Sequence "%f" on the filter command line is replaced with the name of
+the file the filter is working on. A filter might use this in keyword
+substitution. For example:
+
+------------------------
+[filter "p4"]
+ clean = git-p4-filter --clean %f
+ smudge = git-p4-filter --smudge %f
+------------------------
+
Interaction between checkin/checkout attributes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^