git-sh-setup: make require_clean_work_tree part of the interface

92c62a3 (Porcelain scripts: Rewrite cryptic "needs update" error
message, 2010-10-19) refactored git's own checking to a function in
git-sh-setup.  This is a very useful thing for script writers, so
document it.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-sh-setup.txt

@@ -62,6 +62,16 @@ require_work_tree::
 	if so.  Used by scripts that require working tree
 	(e.g. `checkout`).
 
+require_clean_work_tree <action> [<hint>]::
+	checks that the working tree and index associated with the
+	repository have no uncommitted changes to tracked files.
+	Otherwise it emits an error message of the form `Cannot
+	<action>: <reason>. <hint>`, and dies.  Example:
++
+----------------
+require_clean_work_tree rebase "Please commit or stash them."
+----------------
+
 get_author_ident_from_commit::
 	outputs code for use with eval to set the GIT_AUTHOR_NAME,
 	GIT_AUTHOR_EMAIL and GIT_AUTHOR_DATE variables for a given commit.