bookmarks: don't allow name to contain whitespaces only

hgext/bookmarks.py

@@ -136,6 +136,9 b' def bookmark(ui, repo, mark=None, rev=No'
         if "\n" in mark:
             raise util.Abort(_("bookmark name cannot contain newlines"))
         mark = mark.strip()
+        if not mark:
+            raise util.Abort(_("bookmark names cannot consist entirely of "
+                               "whitespace"))
         if mark in marks and not force:
             raise util.Abort(_("a bookmark of the same name already exists"))
         if ((mark in repo.branchtags() or mark == repo.dirstate.branch())

tests/test-bookmarks

@@ -100,4 +100,7 b' hg bookmark'
 echo % revision but no bookmark name
 hg bookmark -r .
 
+echo % bookmark name with whitespace only
+hg bookmark ' '
+
 true

tests/test-bookmarks.out

@@ -74,3 +74,5 b' abort: a bookmark of the same name alrea'
  * x  y                      2:0316ce92851d
 % revision but no bookmark name
 abort: bookmark name required
+% bookmark name with whitespace only
+abort: bookmark names cannot consist entirely of whitespace