Build man pages; spec cleanups

Build man pages; spec cleanups

This patch adds the git man pages to the 'git-core' package, which fixes
the git-buildpackage feature checks for various git commands. It also
fixes git-help(1), which expects the man pages to exist.

Other details of the patch:

- Depend on asciidoc/xmlto, required for building the man pages
- Remove documentation in HTML and text formats (duplicates man pages)
- spec cleanup: Use %{_datadir} instead of /usr/share
- spec cleanup: Remove %defattr macros that are added by default

Change-Id: If1b01506e1bd80e5da2e7a9776f8ec3e01cbbea2