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/regex/regex.h

@@ -41,6 +41,11 @@
 extern "C" {
 #endif
 
+#define regcomp git_regcomp
+#define regexec git_regexec
+#define regerror git_regerror
+#define regfree git_regfree
+
 /* The following two types have to be signed and unsigned integer type
    wide enough to hold a value of a pointer.  For most ANSI compilers
    ptrdiff_t and size_t should be likely OK.  Still size of these two
@@ -322,7 +327,7 @@ typedef enum
   /* POSIX regcomp return error codes.  (In the order listed in the
      standard.)  */
   REG_BADPAT,		/* Invalid pattern.  */
-  REG_ECOLLATE,		/* Inalid collating element.  */
+  REG_ECOLLATE,		/* Invalid collating element.  */
   REG_ECTYPE,		/* Invalid character class name.  */
   REG_EESCAPE,		/* Trailing backslash.  */
   REG_ESUBREG,		/* Invalid back reference.  */