Fix git_commit_dates() to handle (obvious) duplicates

Since git show will suppress duplicate results, change
git_commit_dates() to detect that situation and repair the damage.

Though currently, it can only handle cases where the duplicates are
obvious, i.e. have the same exact string in the argument list.  It
won't be able to handle cases where two arguments differ, but resolve
to the same underlying hash.  For our current purposes, that should be
fine since we only pass in hashes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Move VFS cp() to git.py; handle repodir changes

Move the VFS's cp() to git.py and reset the cp() CatPipe whenever the
global repodir changes.

Previously, if two different lib/bup/t/ tests (for example) needed to
use two different repositories, and both happened to indirectly call
cp(), the second test would end up with a CatPipe() connected to the
wrong repository.

In the longer run, we may want to consider further cleanup here, but
this should fix the immediate problem without too much risk.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Use CatPipe, not show, in git_commit_dates()

Import the commit parser from the experimental bup-get branch and use
it along with CatPipe to produce the git commit dates in
git_commit_dates().

This appears to resolve the performance problem with real archives
that was introduced by the use of "git show -s --pretty=format:%ct
..." (cf. 00ba9fb811e71bb6182b9379461bc6b493e3b7a4), and is still much
faster than 0.25 for at least a 1000 branch synthetic repository here.

Thanks to Gabriel Filion <gabster@lelutin.ca> for reporting the
problem, and to him and Patrick Rouleau <prouleau72@gmail.com> for
helping test the solution.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Don't call _mksubs() in VFS Node nlink()

The VFS Node nlink() code calls _mksubs(), but then just returns 1.
It looks like that may be unnecessary work, so stop doing it.

Thanks to William Stein <wstein@gmail.com> for pointing it out.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>