Add git-zip-tree

In the Windows world ZIP files are better supported than tar files.
Windows even includes built-in support for ZIP files nowadays.

git-zip-tree is similar to git-tar-tree; it creates ZIP files out of
git trees.  It stores the commit ID (if available) in a ZIP file comment
which can be extracted by unzip.

There's still quite some room for improvement: this initial version
supports no symlinks, calls write() way too often (three times per file)
and there is no unit test.

[jc: with a minor typefix to avoid void* arithmetic]

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-zip-tree.txt

@@ -0,0 +1,67 @@
+git-zip-tree(1)
+===============
+
+NAME
+----
+git-zip-tree - Creates a ZIP archive of the files in the named tree
+
+
+SYNOPSIS
+--------
+'git-zip-tree' [-0|...|-9] <tree-ish> [ <base> ]
+
+DESCRIPTION
+-----------
+Creates a ZIP archive containing the tree structure for the named tree.
+When <base> is specified it is added as a leading path to the files in the
+generated ZIP archive.
+
+git-zip-tree behaves differently when given a tree ID versus when given
+a commit ID or tag ID.  In the first case the current time is used as
+modification time of each file in the archive.  In the latter case the
+commit time as recorded in the referenced commit object is used instead.
+Additionally the commit ID is stored as an archive comment.
+
+Currently git-zip-tree can handle only files and directories, symbolic
+links are not supported.
+
+OPTIONS
+-------
+
+-0::
+	Store the files instead of deflating them.
+
+-9::
+	Highest and slowest compression level.  You can specify any
+	number from 1 to 9 to adjust compression speed and ratio.
+
+<tree-ish>::
+	The tree or commit to produce ZIP archive for.  If it is
+	the object name of a commit object.
+
+<base>::
+	Leading path to the files in the resulting ZIP archive.
+
+EXAMPLES
+--------
+git zip-tree v1.4.0 git-1.4.0 >git-1.4.0.zip::
+
+	Create a ZIP file for v1.4.0 release.
+
+git zip-tree HEAD:Documentation/ git-docs >docs.zip::
+
+	Put everything in the current head's Documentation/ directory
+	into 'docs.zip', with the prefix 'git-docs/'.
+
+Author
+------
+Written by Rene Scharfe.
+
+Documentation
+--------------
+Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
+
+GIT
+---
+Part of the gitlink:git[7] suite
+

Makefile

@@ -292,7 +292,8 @@ BUILTIN_OBJS = \
 	builtin-update-ref.o \
 	builtin-upload-tar.o \
 	builtin-verify-pack.o \
-	builtin-write-tree.o
+	builtin-write-tree.o \
+	builtin-zip-tree.o
 
 GITLIBS = $(LIB_FILE) $(XDIFF_LIB)
 LIBS = $(GITLIBS) -lz