Improve C and Elisp Git diff hunk headers

For discussion, see the following threads:
https://lists.gnu.org/archive/html/emacs-devel/2019-03/msg00457.html
https://lists.gnu.org/archive/html/emacs-devel/2019-05/msg00369.html

* autogen.sh: Extend the built-in Git xfuncname pattern 'cpp' to
match preprocessor and DEFUN macros, and the 'elisp' pattern to
match cl-lib and other top-level defuns.
parent 4498e5a1
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