t/perf: handle worktrees as test repos

t/perf: handle worktrees as test repos

The perf suite gets confused when test_perf_default_repo is pointed at a
worktree (which includes when it is run from within a worktree at all,
since the default is to use the current repository).

Here's an example:

  $ git worktree add ~/foo
  Preparing worktree (new branch 'foo')
  HEAD is now at 328c109303 The eighth batch
  $ cd ~/foo
  $ make
  [...build output...]
  $ cd t/perf
  $ ./p0000-perf-lib-sanity.sh -v -i
  [...]
  perf 1 - test_perf_default_repo works:
  running:
  	foo=$(git rev-parse HEAD) &&
  	test_export foo

  fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
  Use '--' to separate paths from revisions, like this:
  'git <command> [<revision>...] -- [<file>...]'

The problem is that we didn't copy all of the necessary files from the
source repository (in this case we got HEAD, but we have no refs!). We
discover the git-dir with "rev-parse --git-dir", but this points to the
worktree's partial repository in .../.git/worktrees/foo.

That partial repository has a "commondir" file which points to the main
repository, where the actual refs are stored, but we don't copy it. This
is the correct thing to do, though! If we did copy it, then our scratch
test repo would be pointing back to the original main repo, and any ref
updates we made in the tests would impact that original repo.

Instead, we need to either:

  1. Make a scratch copy of the original main repo (in addition to the
     worktree repo), and point the scratch worktree repo's commondir at
     it. This preserves the original relationship, but it's doubtful any
     script really cares (if they are testing worktree performance,
     they'd probably make their own worktrees). And it's trickier to get
     right.

  2. Collapse the main and worktree repos into a single scratch repo.
     This can be done by copying everything from both, preferring any
     files from the worktree repo.

This patch does the second one. With this applied, the example above
results in p0000 running successfully.

Reported-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>