cppcheck: handle --diff-filter option

Add the possibility to the cppcheck script to pass git's
`--diff-filter` option.
Also improve comments a bit.

dist/tools/cppcheck/check.sh

@@ -3,18 +3,28 @@
 BRANCH=${1}
 FILEREGEX='\.([sScHh]|cpp)$'
 
-# if branch is an option, unset branch, consume parameter otherwise
+# If no branch but an option is given, unset BRANCH.
+# Otherwise, consume this parameter.
 if echo "${BRANCH}" | grep -q '^-'; then
     BRANCH=""
 else
     shift 1
 fi
 
+# If the --diff-filter option is given, consume this parameter.
+# Set the default DIFFFILTER option otherwise.
+DIFFFILTER="${1}"
+if echo "${DIFFFILTER}" | grep -q '^--diff-filter='; then
+    shift 1
+else
+    DIFFFILTER="--diff-filter=ACMR"
+fi
+
 # select either all or only touched-in-branch files, filter through FILEREGEX
 if [ -z "${BRANCH}" ]; then
     FILES="$(git ls-tree -r --full-tree --name-only HEAD | grep -E ${FILEREGEX})"
 else
-    FILES="$(git diff --diff-filter=ACMR --name-only ${BRANCH} | grep -E ${FILEREGEX})"
+    FILES="$(git diff ${DIFFFILTER} --name-only ${BRANCH} | grep -E ${FILEREGEX})"
 fi
 
 if [ -z "${FILES}" ]; then