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/contrib/completion/git-completion.zsh

@@ -11,8 +11,9 @@
 #
 #  zstyle ':completion:*:*:git:*' script ~/.git-completion.zsh
 #
-# The recommended way to install this script is to copy to '~/.zsh/_git', and
-# then add the following to your ~/.zshrc file:
+# The recommended way to install this script is to make a copy of it in
+# ~/.zsh/ directory as ~/.zsh/git-completion.zsh and then add the following
+# to your ~/.zshrc file:
 #
 #  fpath=(~/.zsh $fpath)
 
@@ -149,9 +150,11 @@ __git_zsh_cmd_common ()
 	push:'update remote refs along with associated objects'
 	rebase:'forward-port local commits to the updated upstream head'
 	reset:'reset current HEAD to the specified state'
+	restore:'restore working tree files'
 	rm:'remove files from the working tree and from the index'
 	show:'show various types of objects'
 	status:'show the working tree status'
+	switch:'switch branches'
 	tag:'create, list, delete or verify a tag object signed with GPG')
 	_describe -t common-commands 'common commands' list && _ret=0
 }

third_party/git/contrib/completion/git-prompt.sh

@@ -70,6 +70,15 @@
 # state symbols by setting GIT_PS1_STATESEPARATOR. The default separator
 # is SP.
 #
+# When there is an in-progress operation such as a merge, rebase,
+# revert, cherry-pick, or bisect, the prompt will include information
+# related to the operation, often in the form "|<OPERATION-NAME>".
+#
+# When the repository has a sparse-checkout, a notification of the form
+# "|SPARSE" will be included in the prompt.  This can be shortened to a
+# single '?' character by setting GIT_PS1_COMPRESSSPARSESTATE, or omitted
+# by setting GIT_PS1_OMITSPARSESTATE.
+#
 # By default, __git_ps1 will compare HEAD to your SVN upstream if it can
 # find one, or @{upstream} otherwise.  Once you have set
 # GIT_PS1_SHOWUPSTREAM, you can override it on a per-repository basis by
@@ -421,6 +430,13 @@ __git_ps1 ()
 		return $exit
 	fi
 
+	local sparse=""
+	if [ -z "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
+	   [ -z "${GIT_PS1_OMITSPARSESTATE}" ] &&
+	   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
+		sparse="|SPARSE"
+	fi
+
 	local r=""
 	local b=""
 	local step=""
@@ -429,11 +445,7 @@ __git_ps1 ()
 		__git_eread "$g/rebase-merge/head-name" b
 		__git_eread "$g/rebase-merge/msgnum" step
 		__git_eread "$g/rebase-merge/end" total
-		if [ -f "$g/rebase-merge/interactive" ]; then
-			r="|REBASE-i"
-		else
-			r="|REBASE-m"
-		fi
+		r="|REBASE"
 	else
 		if [ -d "$g/rebase-apply" ]; then
 			__git_eread "$g/rebase-apply/next" step
@@ -496,6 +508,7 @@ __git_ps1 ()
 	local i=""
 	local s=""
 	local u=""
+	local h=""
 	local c=""
 	local p=""
 
@@ -528,6 +541,11 @@ __git_ps1 ()
 			u="%${ZSH_VERSION+%}"
 		fi
 
+		if [ -n "${GIT_PS1_COMPRESSSPARSESTATE}" ] &&
+		   [ "$(git config --bool core.sparseCheckout)" = "true" ]; then
+			h="?"
+		fi
+
 		if [ -n "${GIT_PS1_SHOWUPSTREAM-}" ]; then
 			__git_ps1_show_upstream
 		fi
@@ -546,8 +564,8 @@ __git_ps1 ()
 		b="\${__git_ps1_branch_name}"
 	fi
 
-	local f="$w$i$s$u"
-	local gitstring="$c$b${f:+$z$f}$r$p"
+	local f="$h$w$i$s$u"
+	local gitstring="$c$b${f:+$z$f}${sparse}$r$p"
 
 	if [ $pcmode = yes ]; then
 		if [ "${__git_printf_supports_v-}" != yes ]; then