git: Add metadata to the result of git archive

When building Xen from a source tarball, commit information is usually lost,
especially if the tarball was generated from a tag.

Have `git archive` automatically fill in metadata at the point of creating the
archive, which is especially useful when using web snapshot links such as:

  http://xenbits.xen.org/gitweb/?p=xen.git;a=snapshot;h=HEAD;sf=tgz

to obtain the tarball.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>

.gitarchive-info

@@ -0,0 +1,2 @@
+Changeset: $Format:%H$
+Commit date: $Format:%cD$