tests: kernel-doc: handle pull requests better

For pull requests, it only tests files changed during the
merge commit itself, which is a bit weird, especially as
the automatic merge won't be able to resolve conflicts.
Use "git diff" instead of "git show" to build the list of
changed files.

Signed-off-by: Johannes Berg <[email protected]>

Author: jmberg

tests/patch/kdoc/kdoc.sh

@@ -8,7 +8,7 @@ tmpfile_o=$(mktemp)
 tmpfile_n=$(mktemp)
 rc=0
 
-files=$(git show --pretty="" --name-only HEAD)
+files=$(git diff HEAD^ --pretty= --name-only)
 
 HEAD=$(git rev-parse HEAD)