fetchFromGitiles: init

This has the same motivation as fetchFromGitHub/fetchFromGitLab --
it's cheaper to download a tarball of a single revision than it is to
download a whole history.

I could have gone with domain/group/repo, like fetchFromGitLab, but it
would have made implementation more difficult, and this syntax means
it's a drop-in replacement for fetchgit, so I decided it wasn't worth
it.

doc/builders/fetchers.xml

@@ -105,6 +105,17 @@ stdenv.mkDerivation {
     </para>
    </listitem>
   </varlistentry>
+  <varlistentry>
+   <term>
+    <literal>fetchFromGitiles</literal>
+   </term>
+   <listitem>
+    <para>
+     This is used with Gitiles repositories. The arguments expected
+     are similar to fetchgit.
+    </para>
+   </listitem>
+  </varlistentry>
   <varlistentry>
    <term>
     <literal>fetchFromBitbucket</literal>