cmake: support for testing git when building out of the source tree

cmake: support for testing git when building out of the source tree

This patch allows git to be tested when performin out of source builds.

This involves changing GIT_BUILD_DIR in t/test-lib.sh to point to the
build directory. Also some miscellaneous copies from the source directory
to the build directory.
The copies are:
t/chainlint.sed needed by a bunch of test scripts
po/is.po needed by t0204-gettext-rencode-sanity
mergetools/tkdiff needed by t7800-difftool
contrib/completion/git-prompt.sh needed by t9903-bash-prompt
contrib/completion/git-completion.bash needed by t9902-completion
contrib/svn-fe/svnrdump_sim.py needed by t9020-remote-svn

NOTE: t/test-lib.sh is only modified when tests are run not during
the build or configure.
The trash directory is still srcdir/t

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