doc: add some crossrefs between manual pages

In particular, git-fast-import and -export link to each
other, and gitremote-helpers links to existing remote
helpers, and vice versa. Also link to fast-import from the
remote helper spec, as this is relevant for remote helpers
using the fast-import format.

Signed-off-by: Max Horn <max@quendi.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-fast-import.txt b/Documentation/git-fast-import.txt
index 377eeaa..f71fb01 100644
--- a/Documentation/git-fast-import.txt
+++ b/Documentation/git-fast-import.txt
@@ -1441,6 +1441,10 @@
 import in progress, at the cost of some added running time and worse
 compression.
 
+SEE ALSO
+--------
+linkgit:git-fast-export[1]
+
 GIT
 ---
 Part of the linkgit:git[1] suite