revert(3p/git): Revert merge of git upstream at v2.26.2

This causes cgit to serve error pages, which is undesirable.

This reverts commit 5229c9b232, reversing
changes made to f2b211131f.

third_party/git/Documentation/git-clean.txt

@@ -26,20 +26,18 @@ are affected.
 OPTIONS
 -------
 -d::
-	Normally, when no <path> is specified, git clean will not
-	recurse into untracked directories to avoid removing too much.
-	Specify -d to have it recurse into such directories as well.
-	If any paths are specified, -d is irrelevant; all untracked
-	files matching the specified paths (with exceptions for nested
-	git directories mentioned under `--force`) will be removed.
+	Remove untracked directories in addition to untracked files.
+	If an untracked directory is managed by a different Git
+	repository, it is not removed by default.  Use -f option twice
+	if you really want to remove such a directory.
 
 -f::
 --force::
 	If the Git configuration variable clean.requireForce is not set
 	to false, 'git clean' will refuse to delete files or directories
-	unless given -f or -i.  Git will refuse to modify untracked
-	nested git repositories (directories with a .git subdirectory)
-	unless a second -f is given.
+	unless given -f, -n or -i. Git will refuse to delete directories
+	with .git sub directory or file unless a second -f
+	is given.
 
 -i::
 --interactive::