refactor(memline): distinguish mutating uses of ml_get_buf()

ml_get_buf() takes a third parameters to indicate whether the
caller wants to mutate the memline data in place. However
the vast majority of the call sites is using this function
just to specify a buffer but without any mutation. This makes
it harder to grep for the places which actually perform mutation.

Solution: Remove the bool param from ml_get_buf(). it now works
like ml_get() except for a non-current buffer. Add a new
ml_get_buf_mut() function for the mutating use-case, which can
be grepped along with the other ml_replace() etc functions which
can modify the memline.

src/nvim/lua/stdlib.c

@@ -105,7 +105,7 @@ static int regex_match_line(lua_State *lstate)
     return luaL_error(lstate, "invalid row");
   }
 
-  char *line = ml_get_buf(buf, rownr + 1, false);
+  char *line = ml_get_buf(buf, rownr + 1);
   size_t len = strlen(line);
 
   if (start < 0 || (size_t)start > len) {