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/compat/obstack.h

@@ -79,7 +79,7 @@ change its address during its lifetime.
 When the chars burst over a chunk boundary, we allocate a larger
 chunk, and then copy the partly formed object from the end of the old
 chunk to the beginning of the new larger chunk.  We then carry on
-accreting characters to the end of the object as we normally would.
+accrediting characters to the end of the object as we normally would.
 
 A special macro is provided to add a single char at a time to a
 growing object.  This allows the use of register variables, which
@@ -135,8 +135,10 @@ extern "C" {
    alignment relative to 0.  */
 
 #define __PTR_ALIGN(B, P, A)						    \
-  __BPTR_ALIGN (sizeof (PTR_INT_TYPE) < sizeof (void *) ? (B) : (char *) 0, \
-		P, A)
+  (sizeof (PTR_INT_TYPE) < sizeof(void *) ?                                 \
+   __BPTR_ALIGN((B), (P), (A)) :                                            \
+   (void *)__BPTR_ALIGN((PTR_INT_TYPE)(void *)0, (PTR_INT_TYPE)(P), (A))            \
+  )
 
 #include <string.h>