The HTTP server side code now uses the same approach that the smart
HTTP client code uses when preparing a request body.  The payload
is streamed into a TemporaryBuffer of limited size.  If the entire
data fits, its compressed with gzip if the user agent supports that,
and a Content-Length header is used to transmit the fixed length
body to the peer.  If however the data overflows the limited memory
segment, its streamed uncompressed to the peer.

One might initially think that larger contents which overflow
the buffer should also be compressed, rather than sent raw, since
they were deemed "large".  But usually these larger contents are
actually a pack file which has been already heavily compressed by
Git specific routines.  Trying to deflate that with gzip is probably
going to take up more space, not less, so the compression overhead
isn't worthwhile.

This buffer and compress optimization helps repositories with a
large number of references, as their text based advertisements
compress well. For example jgit's own native repository currently
requires 32,628 bytes for its full advertisement of 489 references.
Most repositories have fewer references, and thus could compress
their entire response in one buffer.

Change-Id: I790609c9f763339e0a1db9172aa570e29af96f42
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Clients can request smart push support by examining the info/refs URL
with the service parameter set to the magic git-receive-pack string:

  GET /$GIT_DIR/info/refs?service=git-receive-pack HTTP/1.1

The response is formatted with the receive pack capabilities, using
the standard packet line formatter.  A special header block is put
in front of the standard receive-pack advertisement to let clients
know the service was recognized and is supported.

If the requested service is disabled an authorization status code is
returned, allowing the user agent to retry once they have obtained
credentials from a human, in case authentication is required by
the configured ReceivePackFactory implementation.

Change-Id: Ie4f6e0c7b68a68ec4b7cdd5072f91dd406210d4f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

This is a simple HTTP server that provides the minimum server side
support required for dumb (non-git aware) transport clients.

We produce the info/refs and objects/info/packs file on the fly
from the local repository state, but otherwise serve data as raw
files from the on-disk structure.

In the future we could better optimize the FileSender class and the
servlets that use it to take advantage of direct file to network
APIs in more advanced servlet containers like Jetty.

Our glue package borrows the idea of a micro embedded DSL from
Google Guice and uses it to configure a collection of Filters
and HttpServlets, all of which are matched against requests using
regular expressions.  If a subgroup exists in the pattern, it is
extracted and used for the path info component of the request.

Change-Id: Ia0f1a425d07d035e344ae54faf8aeb04763e7487
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Per CQ 3448 this is the initial contribution of the JGit project
to eclipse.org.  It is derived from the historical JGit repository
at commit 3a2dd9921c.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>