fix(diff): use mmfile_t in linematch

Problem:

Linematch used to use strchr to navigate a string, however strchr does
not supoprt embedded NULs.

Solution:

Use `mmfile_t` instead of `char *` in linematch and introduce `strnchr()`.

Also remove heap allocations from `matching_char_iwhite()`

Fixes: #30505

src/clint.py

@@ -881,6 +881,7 @@ def CheckIncludes(filename, lines, error):
             "nvim/func_attr.h",
             "termkey/termkey.h",
             "vterm/vterm.h",
+            "xdiff/xdiff.h",
             ]
 
     for i in check_includes_ignore: