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-show-index.adoc

@@ -0,0 +1,52 @@
+git-show-index(1)
+=================
+
+NAME
+----
+git-show-index - Show packed archive index
+
+
+SYNOPSIS
+--------
+[verse]
+'git show-index' [--object-format=<hash-algorithm>]
+
+
+DESCRIPTION
+-----------
+Read the `.idx` file for a Git packfile (created with
+linkgit:git-pack-objects[1] or linkgit:git-index-pack[1]) from the
+standard input, and dump its contents. The output consists of one object
+per line, with each line containing two or three space-separated
+columns:
+
+  - the first column is the offset in bytes of the object within the
+    corresponding packfile
+
+  - the second column is the object id of the object
+
+  - if the index version is 2 or higher, the third column contains the
+    CRC32 of the object data
+
+The objects are output in the order in which they are found in the index
+file, which should be (in a correctly constructed file) sorted by object
+id.
+
+Note that you can get more information on a packfile by calling
+linkgit:git-verify-pack[1]. However, as this command considers only the
+index file itself, it's both faster and more flexible.
+
+OPTIONS
+-------
+
+--object-format=<hash-algorithm>::
+	Specify the given object format (hash algorithm) for the index file.  The
+	valid values are 'sha1' and (if enabled) 'sha256'.  The default is the
+	algorithm for the current repository (set by `extensions.objectFormat`), or
+	'sha1' if no value is set or outside a repository..
++
+include::object-format-disclaimer.adoc[]
+
+GIT
+---
+Part of the linkgit:git[1] suite