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/Documentation/CodingGuidelines

@@ -75,7 +75,7 @@ For shell scripts specifically (not exhaustive):
 
  - If you want to find out if a command is available on the user's
    $PATH, you should use 'type <command>', instead of 'which <command>'.
-   The output of 'which' is not machine parseable and its exit code
+   The output of 'which' is not machine parsable and its exit code
    is not reliable across platforms.
 
  - We use POSIX compliant parameter substitutions and avoid bashisms;
@@ -91,16 +91,10 @@ For shell scripts specifically (not exhaustive):
 
    - No shell arrays.
 
-   - No strlen ${#parameter}.
-
    - No pattern replacement ${parameter/pattern/string}.
 
  - We use Arithmetic Expansion $(( ... )).
 
- - Inside Arithmetic Expansion, spell shell variables with $ in front
-   of them, as some shells do not grok $((x)) while accepting $(($x))
-   just fine (e.g. dash older than 0.5.4).
-
  - We do not use Process Substitution <(list) or >(list).
 
  - Do not write control structures on a single line with semicolon.
@@ -203,7 +197,7 @@ For C programs:
    . since early 2012 with e1327023ea, we have been using an enum
      definition whose last element is followed by a comma.  This, like
      an array initializer that ends with a trailing comma, can be used
-     to reduce the patch noise when adding a new identifer at the end.
+     to reduce the patch noise when adding a new identifier at the end.
 
    . since mid 2017 with cbc0f81d, we have been using designated
      initializers for struct (e.g. "struct t v = { .val = 'a' };").
@@ -238,6 +232,18 @@ For C programs:
         while( condition )
 		func (bar+1);
 
+ - Do not explicitly compare an integral value with constant 0 or '\0',
+   or a pointer value with constant NULL.  For instance, to validate that
+   counted array <ptr, cnt> is initialized but has no elements, write:
+
+	if (!ptr || cnt)
+		BUG("empty array expected");
+
+   and not:
+
+	if (ptr == NULL || cnt != 0);
+		BUG("empty array expected");
+
  - We avoid using braces unnecessarily.  I.e.
 
 	if (bla) {
@@ -483,16 +489,11 @@ For Python scripts:
 
  - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
 
- - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
+ - As a minimum, we aim to be compatible with Python 2.7.
 
  - Where required libraries do not restrict us to Python 2, we try to
    also be compatible with Python 3.1 and later.
 
- - When you must differentiate between Unicode literals and byte string
-   literals, it is OK to use the 'b' prefix.  Even though the Python
-   documentation for version 2.6 does not mention this prefix, it has
-   been supported since version 2.6.0.
-
 Error Messages
 
  - Do not end error messages with a full stop.