Introduce nvim namespace: Move files.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

src/nvim/diff.h

@@ -0,0 +1,33 @@
+#ifndef NEOVIM_DIFF_H
+#define NEOVIM_DIFF_H
+
+void diff_buf_delete(buf_T *buf);
+void diff_buf_adjust(win_T *win);
+void diff_buf_add(buf_T *buf);
+void diff_invalidate(buf_T *buf);
+void diff_mark_adjust(linenr_T line1, linenr_T line2, long amount,
+                      long amount_after);
+void ex_diffupdate(exarg_T *eap);
+void ex_diffpatch(exarg_T *eap);
+void ex_diffsplit(exarg_T *eap);
+void ex_diffthis(exarg_T *eap);
+void diff_win_options(win_T *wp, int addbuf);
+void ex_diffoff(exarg_T *eap);
+void diff_clear(tabpage_T *tp);
+int diff_check(win_T *wp, linenr_T lnum);
+int diff_check_fill(win_T *wp, linenr_T lnum);
+void diff_set_topline(win_T *fromwin, win_T *towin);
+int diffopt_changed(void);
+int diffopt_horizontal(void);
+int diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp);
+int diff_infold(win_T *wp, linenr_T lnum);
+void nv_diffgetput(int put);
+void ex_diffgetput(exarg_T *eap);
+int diff_mode_buf(buf_T *buf);
+int diff_move_to(int dir, long count);
+linenr_T diff_get_corresponding_line(buf_T *buf1, linenr_T lnum1,
+                                     buf_T *buf2,
+                                     linenr_T lnum3);
+linenr_T diff_lnum_win(linenr_T lnum, win_T *wp);
+
+#endif  // NEOVIM_DIFF_H