travis-ci: don't store P4 and Git LFS in the working tree

travis-ci: don't store P4 and Git LFS in the working tree

The Clang and GCC 64 bit Linux build jobs download and store the P4
and Git LFS executables under the current directory, which is the
working tree that we are about to build and test.  This means that Git
commands like 'status' or 'ls-files' would list these files as
untracked.  The next commit is about to make sure that there are no
untracked files present after the build, and the downloaded
executables in the working tree are interfering with those upcoming
checks.

Therefore, let's download P4 and Git LFS in the home directory,
outside of the working tree.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>