Some clients coming through proxies may advertise a different
Accept-Encoding, for example "Accept-Encoding: gzip(proxy)".
Matching by substring causes us to identify this as a false positive;
that the client understands gzip encoding and will inflate the
response before reading it.

In this particular case however it doesn't.  Its the reverse proxy
server in front of JGit letting us know the proxy<->JGit link can
be gzip compressed, while the client<->proxy part of the link is not:

  client <-- no gzip --> proxy <-- gzip --> JGit

Use a more standard method of parsing by splitting the value into
tokens, and only using gzip if one of the tokens is exactly the
string "gzip".  Add a unit test to make sure this isn't broken in
the future.

Change-Id: Ib4c40f9db177322c7a2640808a6c10b3c4a73819
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

There is no point in pushing all of the files within the edge
commits into the delta search when making a thin pack.  This floods
the delta search window with objects that are unlikely to be useful
bases for the objects that will be written out, resulting in lower
data compression and higher transfer sizes.

Instead observe the path of a tree or blob that is being pushed
into the outgoing set, and use that path to locate up to WINDOW
ancestor versions from the edge commits.  Push only those objects
into the edgeObjects set, reducing the number of objects seen by the
search window.  This allows PackWriter to only look at ancestors
for the modified files, rather than all files in the project.
Limiting the search to WINDOW size makes sense, because more than
WINDOW edge objects will just skip through the window search as
none of them need to be delta compressed.

To further improve compression, sort edge objects into the front
of the window list, rather than randomly throughout.  This puts
non-edges later in the window and gives them a better chance at
finding their base, since they search backwards through the window.

These changes make a significant difference in the thin-pack:

  Before:
    remote: Counting objects: 144190, done
    remote: Finding sources: 100% (50275/50275)
    remote: Getting sizes: 100% (101405/101405)
    remote: Compressing objects: 100% (7587/7587)
    Receiving objects: 100% (50275/50275), 24.67 MiB | 9.90 MiB/s, done.
    Resolving deltas: 100% (40339/40339), completed with 2218 local objects.

    real    0m30.267s

  After:
    remote: Counting objects: 61549, done
    remote: Finding sources: 100% (50275/50275)
    remote: Getting sizes: 100% (18862/18862)
    remote: Compressing objects: 100% (7588/7588)
    Receiving objects: 100% (50275/50275), 11.04 MiB | 3.51 MiB/s, done.
    Resolving deltas: 100% (43160/43160), completed with 5014 local objects.

    real    0m22.170s

The resulting pack is 13.63 MiB smaller, even though it contains the
same exact objects.  82,543 fewer objects had to have their sizes
looked up, which saved about 8s of server CPU time.  2,796 more
objects from the client were used as part of the base object set,
which contributed to the smaller transfer size.

Change-Id: Id01271950432c6960897495b09deab70e33993a9
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Sigend-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

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>