Mark gix-pack vendored izip! doctest ignore

That macro is not exported, and adjusting the doctest so that it
can run successfully (or even compile) might be tricky without a
significant change (or adding dependencies). This fixes a failure
when running doctests for the whole workspace (as CI now includes).

This change is not "user-facing", as the macro is not part of the
public interface of `gix_pack`. Its documentation comment doesn't
appear https://docs.rs/gix-pack/0.54.0/gix_pack/index/index.html.
Nonetheless, this adds a note to clarify the change, to avoid
attributing it to the original code from which this was vendored.

Author: EliahKagan

gix-pack/src/index/mod.rs

@@ -1,5 +1,5 @@
 //! an index into the pack file
-//!
+
 /// From itertools
 /// Create an iterator running multiple iterators in lockstep.
 ///
@@ -21,7 +21,7 @@
 ///
 /// [`multizip`]: fn.multizip.html
 ///
-/// ```
+/// ```ignore
 /// # use itertools::izip;
 /// #
 /// # fn main() {
@@ -37,6 +37,9 @@
 /// assert_eq!(results, [0 + 3, 10 + 7, 29, 36]);
 /// # }
 /// ```
+///
+/// (The above is vendored from [itertools](https://github.com/rust-itertools/itertools),
+/// including the original doctest, though it has been marked `ignore` here.)
 macro_rules! izip {
     // @closure creates a tuple-flattening closure for .map() call. usage:
     // @closure partial_pattern => partial_tuple , rest , of , iterators