ignore: note debugignore on ignore man page

It took me a long time to find debugignore. I found the ignore man page
quickly. This change adds a debugging section to the ignore man page
letting people know there is a debug command.

mercurial/helptext/hgignore.txt

@@ -1,97 +1,103 b''
 Synopsis
 ========
 
 The Mercurial system uses a file called ``.hgignore`` in the root
 directory of a repository to control its behavior when it searches
 for files that it is not currently tracking.
 
 Description
 ===========
 
 The working directory of a Mercurial repository will often contain
 files that should not be tracked by Mercurial. These include backup
 files created by editors and build products created by compilers.
 These files can be ignored by listing them in a ``.hgignore`` file in
 the root of the working directory. The ``.hgignore`` file must be
 created manually. It is typically put under version control, so that
 the settings will propagate to other repositories with push and pull.
 
 An untracked file is ignored if its path relative to the repository
 root directory, or any prefix path of that path, is matched against
 any pattern in ``.hgignore``.
 
 For example, say we have an untracked file, ``file.c``, at
 ``a/b/file.c`` inside our repository. Mercurial will ignore ``file.c``
 if any pattern in ``.hgignore`` matches ``a/b/file.c``, ``a/b`` or ``a``.
 
 In addition, a Mercurial configuration file can reference a set of
 per-user or global ignore files. See the ``ignore`` configuration
 key on the ``[ui]`` section of :hg:`help config` for details of how to
 configure these files.
 
 To control Mercurial's handling of files that it manages, many
 commands support the ``-I`` and ``-X`` options; see
 :hg:`help <command>` and :hg:`help patterns` for details.
 
 Files that are already tracked are not affected by .hgignore, even
 if they appear in .hgignore. An untracked file X can be explicitly
 added with :hg:`add X`, even if X would be excluded by a pattern
 in .hgignore.
 
 Syntax
 ======
 
 An ignore file is a plain text file consisting of a list of patterns,
 with one pattern per line. Empty lines are skipped. The ``#``
 character is treated as a comment character, and the ``\`` character
 is treated as an escape character.
 
 Mercurial supports several pattern syntaxes. The default syntax used
 is Python/Perl-style regular expressions.
 
 To change the syntax used, use a line of the following form::
 
   syntax: NAME
 
 where ``NAME`` is one of the following:
 
 ``regexp``
   Regular expression, Python/Perl syntax.
 ``glob``
   Shell-style glob.
 ``rootglob``
   A variant of ``glob`` that is rooted (see below).
 
 The chosen syntax stays in effect when parsing all patterns that
 follow, until another syntax is selected.
 
 Neither ``glob`` nor regexp patterns are rooted. A glob-syntax
 pattern of the form ``*.c`` will match a file ending in ``.c`` in any
 directory, and a regexp pattern of the form ``\.c$`` will do the
 same. To root a regexp pattern, start it with ``^``. To get the same
 effect with glob-syntax, you have to use ``rootglob``.
 
 Subdirectories can have their own .hgignore settings by adding
 ``subinclude:path/to/subdir/.hgignore`` to the root ``.hgignore``. See
 :hg:`help patterns` for details on ``subinclude:`` and ``include:``.
 
 .. note::
 
   Patterns specified in other than ``.hgignore`` are always rooted.
   Please see :hg:`help patterns` for details.
 
 Example
 =======
 
 Here is an example ignore file. ::
 
   # use glob syntax.
   syntax: glob
 
   *.elc
   *.pyc
   *~
 
   # switch to regexp syntax.
   syntax: regexp
   ^\.pc/
+
+Debugging
+=========
+
+Use the ``debugignore`` command to see if and why a file is ignored, or to
+see the combined ignore pattern. See :hg:`help debugignore` for details.