upload-pack: optionally allow fetching reachable sha1

With uploadpack.allowReachableSHA1InWant configuration option set on the
server side, "git fetch" can make a request with a "want" line that names
an object that has not been advertised (likely to have been obtained out
of band or from a submodule pointer). Only objects reachable from the
branch tips, i.e. the union of advertised branches and branches hidden by
transfer.hideRefs, will be processed. Note that there is an associated
cost of having to walk back the history to check the reachability.

This feature can be used when obtaining the content of a certain commit,
for which the sha1 is known, without the need of cloning the whole
repository, especially if a shallow fetch is used. Useful cases are e.g.
repositories containing large files in the history, fetching only the
needed data for a submodule checkout, when sharing a sha1 without telling
which exact branch it belongs to and in Gerrit, if you think in terms of
commits instead of change numbers. (The Gerrit case has already been
solved through allowTipSHA1InWant as every Gerrit change has a ref.)

Signed-off-by: Fredrik Medley <fredrik.medley@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/http-protocol.txt

@@ -319,7 +319,8 @@ Servers SHOULD support all capabilities defined here.
 Clients MUST send at least one "want" command in the request body.
 Clients MUST NOT reference an id in a "want" command which did not
 appear in the response obtained through ref discovery unless the
-server advertises capability `allow-tip-sha1-in-want`.
+server advertises capability `allow-tip-sha1-in-want` or
+`allow-reachable-sha1-in-want`.
 
   compute_request   =  want_list
 		       have_list

Documentation/technical/protocol-capabilities.txt

@@ -260,6 +260,13 @@ If the upload-pack server advertises this capability, fetch-pack may
 send "want" lines with SHA-1s that exist at the server but are not
 advertised by upload-pack.
 
+allow-reachable-sha1-in-want
+----------------------------
+
+If the upload-pack server advertises this capability, fetch-pack may
+send "want" lines with SHA-1s that exist at the server but are not
+advertised by upload-pack.
+
 push-cert=<nonce>
 -----------------