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/diff-options.txt

@@ -73,6 +73,11 @@ ifndef::git-format-patch[]
 	Synonym for `-p --raw`.
 endif::git-format-patch[]
 
+ifdef::git-log[]
+-t::
+	Show the tree objects in the diff output.
+endif::git-log[]
+
 --indent-heuristic::
 	Enable the heuristic that shifts diff hunk boundaries to make patches
 	easier to read. This is the default.
@@ -441,10 +446,11 @@ endif::git-format-patch[]
 --abbrev[=<n>]::
 	Instead of showing the full 40-byte hexadecimal object
 	name in diff-raw format output and diff-tree header
-	lines, show only a partial prefix.  This is
-	independent of the `--full-index` option above, which controls
-	the diff-patch output format.  Non default number of
-	digits can be specified with `--abbrev=<n>`.
+	lines, show only a partial prefix.
+	In diff-patch output format, `--full-index` takes higher
+	precedence, i.e. if `--full-index` is specified, full blob
+	names will be shown regardless of `--abbrev`.
+	Non default number of digits can be specified with `--abbrev=<n>`.
 
 -B[<n>][/<m>]::
 --break-rewrites[=[<n>][/<m>]]::
@@ -567,13 +573,13 @@ To illustrate the difference between `-S<regex> --pickaxe-regex` and
 file:
 +
 ----
-+    return !regexec(regexp, two->ptr, 1, &regmatch, 0);
++    return frotz(nitfol, two->ptr, 1, 0);
 ...
--    hit = !regexec(regexp, mf2.ptr, 1, &regmatch, 0);
+-    hit = frotz(nitfol, mf2.ptr, 1, 0);
 ----
 +
-While `git log -G"regexec\(regexp"` will show this commit, `git log
--S"regexec\(regexp" --pickaxe-regex` will not (because the number of
+While `git log -G"frotz\(nitfol"` will show this commit, `git log
+-S"frotz\(nitfol" --pickaxe-regex` will not (because the number of
 occurrences of that string did not change).
 +
 Unless `--text` is supplied patches of binary files without a textconv
@@ -643,15 +649,18 @@ ifndef::git-format-patch[]
 -R::
 	Swap two inputs; that is, show differences from index or
 	on-disk file to tree contents.
+endif::git-format-patch[]
 
 --relative[=<path>]::
+--no-relative::
 	When run from a subdirectory of the project, it can be
 	told to exclude changes outside the directory and show
 	pathnames relative to it with this option.  When you are
 	not in a subdirectory (e.g. in a bare repository), you
 	can name which subdirectory to make the output relative
 	to by giving a <path> as an argument.
-endif::git-format-patch[]
+	`--no-relative` can be used to countermand both `diff.relative` config
+	option and previous `--relative`.
 
 -a::
 --text::