pull: Support specifying exact commit to pull via branch@commit

I don't know why we didn't do this a long time ago.  This extends the
pull API to allow grabbing a specific commit, and will set the branch
to it.  There's some support for this in the deploy engine, but there
are a lot of reasons to support it for raw pulls (such as subset
mirroring cases).

In fact I'm thinking we should also have the override-version logic
here too.

NOTE: One thing I debated here is inventing a new syntax on the
command line.  Git doesn't seem to have this functionality (probably
because it'd be rarely used). The '@' character at least doesn't
conflict with anything.

Anyways, I wanted this for some other test cases.  Without this,
writing tests that go between different commits is more awkward as one
must generate the content in one repo, then pull downstream, then
generate more content, then pull again.  But now I can just keep track
of commit IDs and do exactly what I want without synchronizing the
tests.

man/ostree-pull.xml

@@ -111,14 +111,37 @@ Boston, MA 02111-1307, USA.
         <title>Description</title>
 
         <para>
-            Downloads all content corresponding to the provided branch or commit from the given remote.
+	  This command can retrieve just a specific commit, or go all
+	  the way to performing a full mirror of the remote
+	  repository.  If no <literal>BRANCH</literal> is specified,
+	  all branches are retrieved.
         </para>
+
+	<para>
+	  A special syntax in the <literal>@</literal> character allows
+	  specifying a specific commit to retrieve from a branch.  This
+	</para>
+
     </refsect1>
 
     <refsect1>
         <title>Example</title>
 
-        <para><command>$ ostree pull remote_name</command></para>
+        <para><command>$ ostree --repo=repo pull --depth=-1 --mirror remote_name</command></para>
+
+	<para>Perform a complete mirror of the remote.  (This is
+	likely most useful if your repository is also
+	<literal>archive-z2</literal> mode)</para>
+
+        <para><command>$ ostree --repo=repo pull remote_name exampleos/x86_64/standard</command></para>
+
+	<para>Fetch the most recent commit to <literal>exampleos/x86_64/standard</literal>.</para>
+
+        <para><command>$ ostree --repo=repo pull remote_name exampleos/x86_64/standard@98ea6e4f216f2fb4b69fff9b3a44842c38686ca685f3f55dc48c5d3fb1107be4</command></para>
+
+	<para>Download the specific commit starting with
+	<literal>98ea6e</literal> as if it was the latest commit for
+	<literal>exampleos/x86_64/standard</literal>.</para>
     </refsect1>
 
 </refentry>