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-merge-base.txt

@@ -80,9 +80,11 @@ which is reachable from both 'A' and 'B' through the parent relationship.
 
 For example, with this topology:
 
-	         o---o---o---B
-	        /
-	---o---1---o---o---o---A
+....
+	 o---o---o---B
+	/
+---o---1---o---o---o---A
+....
 
 the merge base between 'A' and 'B' is '1'.
 
@@ -90,21 +92,25 @@ Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the
 merge base between 'A' and a hypothetical commit 'M', which is a merge
 between 'B' and 'C'.  For example, with this topology:
 
-	       o---o---o---o---C
-	      /
-	     /   o---o---o---B
-	    /   /
-	---2---1---o---o---o---A
+....
+       o---o---o---o---C
+      /
+     /   o---o---o---B
+    /   /
+---2---1---o---o---o---A
+....
 
 the result of `git merge-base A B C` is '1'.  This is because the
 equivalent topology with a merge commit 'M' between 'B' and 'C' is:
 
 
-	       o---o---o---o---o
-	      /                 \
-	     /   o---o---o---o---M
-	    /   /
-	---2---1---o---o---o---A
+....
+       o---o---o---o---o
+      /                 \
+     /   o---o---o---o---M
+    /   /
+---2---1---o---o---o---A
+....
 
 and the result of `git merge-base A M` is '1'.  Commit '2' is also a
 common ancestor between 'A' and 'M', but '1' is a better common ancestor,
@@ -116,11 +122,13 @@ the best common ancestor of all commits.
 When the history involves criss-cross merges, there can be more than one
 'best' common ancestor for two commits.  For example, with this topology:
 
-	---1---o---A
-	    \ /
-	     X
-	    / \
-	---2---o---o---B
+....
+---1---o---A
+    \ /
+     X
+    / \
+---2---o---o---B
+....
 
 both '1' and '2' are merge-bases of A and B.  Neither one is better than
 the other (both are 'best' merge bases).  When the `--all` option is not given,
@@ -131,18 +139,22 @@ and B is (or at least used to be) to compute the merge base between
 A and B, and check if it is the same as A, in which case, A is an
 ancestor of B.  You will see this idiom used often in older scripts.
 
-	A=$(git rev-parse --verify A)
-	if test "$A" = "$(git merge-base A B)"
-	then
-		... A is an ancestor of B ...
-	fi
+....
+A=$(git rev-parse --verify A)
+if test "$A" = "$(git merge-base A B)"
+then
+	... A is an ancestor of B ...
+fi
+....
 
 In modern git, you can say this in a more direct way:
 
-	if git merge-base --is-ancestor A B
-	then
-		... A is an ancestor of B ...
-	fi
+....
+if git merge-base --is-ancestor A B
+then
+	... A is an ancestor of B ...
+fi
+....
 
 instead.
 
@@ -154,13 +166,15 @@ topic origin/master`, the history of remote-tracking branch
 `origin/master` may have been rewound and rebuilt, leading to a
 history of this shape:
 
-	                 o---B2
-	                /
-	---o---o---B1--o---o---o---B (origin/master)
-	        \
-	         B0
-	          \
-	           D0---D1---D (topic)
+....
+		 o---B2
+		/
+---o---o---B1--o---o---o---B (origin/master)
+	\
+	 B0
+	  \
+	   D0---D1---D (topic)
+....
 
 where `origin/master` used to point at commits B0, B1, B2 and now it
 points at B, and your `topic` branch was started on top of it back
@@ -193,13 +207,15 @@ will find B0, and
 will replay D0, D1 and D on top of B to create a new history of this
 shape:
 
-			 o---B2
-			/
-	---o---o---B1--o---o---o---B (origin/master)
-		\                   \
-		 B0                  D0'--D1'--D' (topic - updated)
-		  \
-		   D0---D1---D (topic - old)
+....
+		 o---B2
+		/
+---o---o---B1--o---o---o---B (origin/master)
+	\                   \
+	 B0                  D0'--D1'--D' (topic - updated)
+	  \
+	   D0---D1---D (topic - old)
+....
 
 A caveat is that older reflog entries in your repository may be
 expired by `git gc`.  If B0 no longer appears in the reflog of the