fix 'make log' with non-default git config format.pretty

suggested by Aaron Jensen <aaronjensen@gmail.com>.

REFMAIL: CAHyO48z0DcoFPC8rCNAL38oxVQtZNKifVd-NEF3sp1EfR-GgxQ@mail.gmail.com

Makefile.am

@@ -51,7 +51,7 @@ $(srcdir)/ChangeLog: log
 log:
 	@test -z "$(srcdir)" || cd $(srcdir) && \
 	 ( ! test -d .git || \
-	   git log --date=iso --log-size --name-only --no-merges | \
+	   git log --pretty=medium --date=iso --log-size --name-only --no-merges | \
 	     perl -e '$(LOG_PL)' > ChangeLog )
 
 cov-scan: clean