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/git-apply.txt

@@ -61,18 +61,18 @@ OPTIONS
 	file and detects errors.  Turns off "apply".
 
 --index::
-	When `--check` is in effect, or when applying the patch
-	(which is the default when none of the options that
-	disables it is in effect), make sure the patch is
-	applicable to what the current index file records.  If
-	the file to be patched in the working tree is not
-	up to date, it is flagged as an error.  This flag also
-	causes the index file to be updated.
+	Apply the patch to both the index and the working tree (or
+	merely check that it would apply cleanly to both if `--check` is
+	in effect). Note that `--index` expects index entries and
+	working tree copies for relevant paths to be identical (their
+	contents and metadata such as file mode must match), and will
+	raise an error if they are not, even if the patch would apply
+	cleanly to both the index and the working tree in isolation.
 
 --cached::
-	Apply a patch without touching the working tree. Instead take the
-	cached data, apply the patch, and store the result in the index
-	without using the working tree. This implies `--index`.
+	Apply the patch to just the index, without touching the working
+	tree. If `--check` is in effect, merely check that it would
+	apply cleanly to the index entry.
 
 --intent-to-add::
 	When applying the patch only to the working tree, mark new