p0006-read-tree-checkout: perf test to time read-tree

p0006-read-tree-checkout: perf test to time read-tree

Created t/perf/repos/many-files.sh to generate large, but
artificial repositories.

Created t/perf/inflate-repo.sh to alter an EXISTING repo
to have a set of large commits.  This can be used to create
a branch with 1M+ files in repositories like git.git or
linux.git, but with more realistic content.  It does this
by making multiple copies of the entire worktree in a series
of sub-directories.

The branch name and ballast structure created by both scripts
match, so either script can be used to generate very large
test repositories for the following perf test.

Created t/perf/p0006-read-tree-checkout.sh to measure
performance on various read-tree, checkout, and update-index
operations.  This test can run using either normal repos or
ones from the above scripts.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>