merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/commit-slab.h

@@ -24,7 +24,12 @@
  * - int *indegree_peek(struct indegree *, struct commit *);
  *
  *   This function is similar to indegree_at(), but it will return NULL
- *   until a call to indegree_at() was made for the commit.
+ *   if the location to store the data associated with the given commit
+ *   has not been allocated yet.
+ *   Note that the location to store the data might have already been
+ *   allocated even if no indegree_at() call has been made for that commit
+ *   yet; in this case this function returns a pointer to a
+ *   zero-initialized location.
  *
  * - void init_indegree(struct indegree *);
  *   void init_indegree_with_stride(struct indegree *, int);
@@ -42,6 +47,16 @@
  *
  *   Call this function before the slab falls out of scope to avoid
  *   leaking memory.
+ *
+ * - void deep_clear_indegree(struct indegree *, void (*free_fn)(int*))
+ *
+ *   Empties the slab, similar to clear_indegree(), but in addition it
+ *   calls the given 'free_fn' for each slab entry to release any
+ *   additional memory that might be owned by the entry (but not the
+ *   entry itself!).
+ *   Note that 'free_fn' might be called even for entries for which no
+ *   indegree_at() call has been made; in this case 'free_fn' is invoked
+ *   with a pointer to a zero-initialized location.
  */
 
 #define define_commit_slab(slabname, elemtype) \