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/executor.c

@@ -1707,7 +1707,7 @@ void ex_luado(exarg_T *const eap)
       break;
     }
     lua_pushvalue(lstate, -1);
-    const char *const old_line = ml_get_buf(curbuf, l, false);
+    const char *const old_line = ml_get_buf(curbuf, l);
     // Get length of old_line here as calling Lua code may free it.
     const size_t old_line_len = strlen(old_line);
     lua_pushstring(lstate, old_line);