git: expand recipe to take advantage of pre-gen'd manpages

git: expand recipe to take advantage of pre-gen'd manpages

These could be created from scratch from git itself, but it
requires asciidoc, xsltproc, python bits and too much other
baggage.  Since the git folks issue a tarball with the manpages
for each release, it is simpler to just go get that.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>