ENH: GIT_WC_INFO with date support for git only repository

To allow the date to be extracted from git only repository, the macro
"GIT_WC_INFO" has been extended to extract the commit date of the last
change.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23162 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Associate extension package name with the date of the last commit.

A similar problem was occurring while building the Slicer core and has
been fixed by r20028[1]. The associated issues were #1974 [2] and #1973 [3].

This commit associate to the package name the date of the latest commit
associated with the extension source repository. In case no repository
has been associated with the extension, the date "0000-00-00" will be
considered.

Thanks to "Chenxi Zhang <chenxizhang@fudan.edu.cn>" for reporting the
problem.

Associated discussion: http://slicer-devel.65872.n3.nabble.com/Problem-in-building-a-extension-tp4029816p4029857.html

Fixes #3356

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=20028
[2] http://www.na-mic.org/Bug/view.php?id=1973
[3] http://www.na-mic.org/Bug/view.php?id=1974

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23163 3bd1e089-480b-0410-8dfb-8563597acbee