doc: document the ui.patch option
This option has been undocumented since
e56c7e05c7e6 (July 2007).
--- a/mercurial/help/config.txt Mon Jun 22 13:48:01 2015 -0700
+++ b/mercurial/help/config.txt Tue Jun 23 13:47:42 2015 -0400
@@ -1451,6 +1451,19 @@
markers is different from the encoding of the merged files,
serious problems may occur.
+``patch``
+ An optional external tool that ``hg import`` and some extensions
+ will use for applying patches. By default Mercurial uses an
+ internal patch utility. The external tool must work as the common
+ Unix ``patch`` program. In particular, it must accept a ``-p``
+ argument to strip patch headers, a ``-d`` argument to specify the
+ current directory, a file name to patch, and a patch file to take
+ from stdin.
+
+ It is possible to specify a patch tool together with extra
+ arguments. For example, setting this option to ``patch --merge``
+ will use the ``patch`` program with its 2-way merge option.
+
``portablefilenames``
Check for portable filenames. Can be ``warn``, ``ignore`` or ``abort``.
Default is ``warn``.