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-log.txt

@@ -15,9 +15,12 @@ DESCRIPTION
 -----------
 Shows the commit logs.
 
-The command takes options applicable to the `git rev-list`
+:git-log: 1
+include::rev-list-description.txt[]
+
+The command takes options applicable to the linkgit:git-rev-list[1]
 command to control what is shown and how, and options applicable to
-the `git diff-*` commands to control how the changes
+the linkgit:git-diff[1] command to control how the changes
 each commit introduces are shown.
 
 
@@ -43,12 +46,16 @@ OPTIONS
 	If no `--decorate-refs` is given, pretend as if all refs were
 	included.  For each candidate, do not use it for decoration if it
 	matches any patterns given to `--decorate-refs-exclude` or if it
-	doesn't match any of the patterns given to `--decorate-refs`.
+	doesn't match any of the patterns given to `--decorate-refs`. The
+	`log.excludeDecoration` config option allows excluding refs from
+	the decorations, but an explicit `--decorate-refs` pattern will
+	override a match in `log.excludeDecoration`.
 
 --source::
 	Print out the ref name given on the command line by which each
 	commit was reached.
 
+--[no-]mailmap::
 --[no-]use-mailmap::
 	Use mailmap file to map author and committer names and email
 	addresses to canonical real names and email addresses. See
@@ -76,8 +83,12 @@ produced by `--stat`, etc.
 	(or the function name regex <funcname>) within the <file>.  You may
 	not give any pathspec limiters.  This is currently limited to
 	a walk starting from a single revision, i.e., you may only
-	give zero or one positive revision arguments.
-	You can specify this option more than once.
+	give zero or one positive revision arguments, and
+	<start> and <end> (or <funcname>) must exist in the starting revision.
+	You can specify this option more than once. Implies `--patch`.
+	Patch output can be suppressed using `--no-patch`, but other diff formats
+	(namely `--raw`, `--numstat`, `--shortstat`, `--dirstat`, `--summary`,
+	`--name-only`, `--name-status`, `--check`) are not currently implemented.
 +
 include::line-range-format.txt[]
 
@@ -103,8 +114,51 @@ include::rev-list-options.txt[]
 
 include::pretty-formats.txt[]
 
-COMMON DIFF OPTIONS
--------------------
+DIFF FORMATTING
+---------------
+
+By default, `git log` does not generate any diff output. The options
+below can be used to show the changes made by each commit.
+
+Note that unless one of `-c`, `--cc`, or `-m` is given, merge commits
+will never show a diff, even if a diff format like `--patch` is
+selected, nor will they match search options like `-S`. The exception is
+when `--first-parent` is in use, in which merges are treated like normal
+single-parent commits (this can be overridden by providing a
+combined-diff option or with `--no-diff-merges`).
+
+-c::
+	With this option, diff output for a merge commit
+	shows the differences from each of the parents to the merge result
+	simultaneously instead of showing pairwise diff between a parent
+	and the result one at a time. Furthermore, it lists only files
+	which were modified from all parents.
+
+--cc::
+	This flag implies the `-c` option and further compresses the
+	patch output by omitting uninteresting hunks whose contents in
+	the parents have only two variants and the merge result picks
+	one of them without modification.
+
+--combined-all-paths::
+	This flag causes combined diffs (used for merge commits) to
+	list the name of the file from all parents.  It thus only has
+	effect when -c or --cc are specified, and is likely only
+	useful if filename changes are detected (i.e. when either
+	rename or copy detection have been requested).
+
+-m::
+	This flag makes the merge commits show the full diff like
+	regular commits; for each merge parent, a separate log entry
+	and diff is generated. An exception is that only diff against
+	the first parent is shown when `--first-parent` option is given;
+	in that case, the output represents the changes the merge
+	brought _into_ the then-current branch.
+
+--diff-merges=off::
+--no-diff-merges::
+	Disable output of diffs for merge commits (default). Useful to
+	override `-m`, `-c`, or `--cc`.
 
 :git-log: 1
 include::diff-options.txt[]