doc: use .adoc extension for AsciiDoc files

We presently use the ".txt" extension for our AsciiDoc files.  While not
wrong, most editors do not associate this extension with AsciiDoc,
meaning that contributors don't get automatic editor functionality that
could be useful, such as syntax highlighting and prose linting.

It is much more common to use the ".adoc" extension for AsciiDoc files,
since this helps editors automatically detect files and also allows
various forges to provide rich (HTML-like) rendering.  Let's do that
here, renaming all of the files and updating the includes where
relevant.  Adjust the various build scripts and makefiles to use the new
extension as well.

Note that this should not result in any user-visible changes to the
documentation.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-write-tree.adoc

@@ -0,0 +1,42 @@
+git-write-tree(1)
+=================
+
+NAME
+----
+git-write-tree - Create a tree object from the current index
+
+
+SYNOPSIS
+--------
+[verse]
+'git write-tree' [--missing-ok] [--prefix=<prefix>/]
+
+DESCRIPTION
+-----------
+Creates a tree object using the current index. The name of the new
+tree object is printed to standard output.
+
+The index must be in a fully merged state.
+
+Conceptually, 'git write-tree' sync()s the current index contents
+into a set of tree files.
+In order to have that match what is actually in your directory right
+now, you need to have done a 'git update-index' phase before you did the
+'git write-tree'.
+
+
+OPTIONS
+-------
+--missing-ok::
+	Normally 'git write-tree' ensures that the objects referenced by the
+	directory exist in the object database.  This option disables this
+	check.
+
+--prefix=<prefix>/::
+	Writes a tree object that represents a subdirectory
+	`<prefix>`.  This can be used to write the tree object
+	for a subproject that is in the named subdirectory.
+
+GIT
+---
+Part of the linkgit:git[1] suite