cmake: installation support for git

cmake: installation support for git

Install the built binaries and scripts using CMake

This is very similar to `make install`.
By default the destination directory(DESTDIR) is /usr/local/ on Linux
To set a custom installation path do this:
cmake `relative-path-to-srcdir`
	-DCMAKE_INSTALL_PREFIX=`preferred-install-path`

Then run `make install`

Signed-off-by: Sibi Siddharthan <sibisiddharthan.github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>