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-for-each-ref.txt

@@ -11,7 +11,7 @@ SYNOPSIS
 'git for-each-ref' [--count=<count>] [--shell|--perl|--python|--tcl]
 		   [(--sort=<key>)...] [--format=<format>] [<pattern>...]
 		   [--points-at=<object>]
-		   (--merged[=<object>] | --no-merged[=<object>])
+		   [--merged[=<object>]] [--no-merged[=<object>]]
 		   [--contains[=<object>]] [--no-contains[=<object>]]
 
 DESCRIPTION
@@ -76,13 +76,11 @@ OPTIONS
 
 --merged[=<object>]::
 	Only list refs whose tips are reachable from the
-	specified commit (HEAD if not specified),
-	incompatible with `--no-merged`.
+	specified commit (HEAD if not specified).
 
 --no-merged[=<object>]::
 	Only list refs whose tips are not reachable from the
-	specified commit (HEAD if not specified),
-	incompatible with `--merged`.
+	specified commit (HEAD if not specified).
 
 --contains[=<object>]::
 	Only list refs which contain the specified commit (HEAD if not
@@ -222,6 +220,8 @@ worktreepath::
 In addition to the above, for commit and tag objects, the header
 field names (`tree`, `parent`, `object`, `type`, and `tag`) can
 be used to specify the value in the header field.
+Fields `tree` and `parent` can also be used with modifier `:short` and
+`:short=<length>` just like `objectname`.
 
 For commit and tag objects, the special `creatordate` and `creator`
 fields will correspond to the appropriate date or name-email-date tuple
@@ -230,14 +230,35 @@ These are intended for working on a mix of annotated and lightweight tags.
 
 Fields that have name-email-date tuple as its value (`author`,
 `committer`, and `tagger`) can be suffixed with `name`, `email`,
-and `date` to extract the named component.
+and `date` to extract the named component.  For email fields (`authoremail`,
+`committeremail` and `taggeremail`), `:trim` can be appended to get the email
+without angle brackets, and `:localpart` to get the part before the `@` symbol
+out of the trimmed email.
+
+The message in a commit or a tag object is `contents`, from which
+`contents:<part>` can be used to extract various parts out of:
+
+contents:size::
+	The size in bytes of the commit or tag message.
+
+contents:subject::
+	The first paragraph of the message, which typically is a
+	single line, is taken as the "subject" of the commit or the
+	tag message.
+	Instead of `contents:subject`, field `subject` can also be used to
+	obtain same results. `:sanitize` can be appended to `subject` for
+	subject line suitable for filename.
+
+contents:body::
+	The remainder of the commit or the tag message that follows
+	the "subject".
+
+contents:signature::
+	The optional GPG signature of the tag.
+
+contents:lines=N::
+	The first `N` lines of the message.
 
-The complete message in a commit and tag object is `contents`.
-Its first line is `contents:subject`, where subject is the concatenation
-of all lines of the commit message up to the first blank line.  The next
-line is `contents:body`, where body is all of the lines after the first
-blank line.  The optional GPG signature is `contents:signature`.  The
-first `N` lines of the message is obtained using `contents:lines=N`.
 Additionally, the trailers as interpreted by linkgit:git-interpret-trailers[1]
 are obtained as `trailers` (or by using the historical alias
 `contents:trailers`).  Non-trailer lines from the trailer block can be omitted
@@ -385,6 +406,11 @@ Note also that multiple copies of an object may be present in the object
 database; in this case, it is undefined which copy's size or delta base
 will be reported.
 
+NOTES
+-----
+
+include::ref-reachability-filters.txt[]
+
 SEE ALSO
 --------
 linkgit:git-show-ref[1]