cd_to_toplevel, which is impossible to do if there is no
working tree.
+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.