Git::SVN::*: avoid premature FileHandle closure

Since b19138b (git-svn: Make it incrementally faster by minimizing temp
files, v1.6.0), git-svn has been using the Git.pm temp_acquire and
temp_release mechanism to avoid unnecessary temp file churn and provide
a speed boost.

However, that change introduced a call to temp_acquire inside the
Git::SVN::Fetcher::close_file function for an 'svn_hash' temp file.
Because an SVN::Pool is active at the time this function is called, if
the Git::temp_acquire function ends up actually creating a new
FileHandle for the temp file (which it will the first time it's called
with the name 'svn_hash') that FileHandle will end up in the SVN::Pool
and should that pool have SVN::Pool::clear called on it that FileHandle
will be closed out from under Git::temp_acquire.

Since the only call site to Git::temp_acquire with the name 'svn_hash'
is inside the close_file function, if an 'svn_hash' temp file is ever
created its FileHandle is guaranteed to be created in the active
SVN::Pool.

This has not been a problem in the past because the SVN::Pool was not
being cleared.  However, since dfa72fdb (git-svn: reload RA every
log-window-size, v2.2.0) the pool has been getting cleared periodically
at which point the FileHandle for the 'svn_hash' temp file gets closed.
Any subsequent calls to Git::temp_acquire for 'svn_hash', however,
succeed without creating/opening a new temporary file since it still has
the now invalid FileHandle in its cache.  Callers that then attempt to
use that FileHandle fail with an error.

We avoid this problem by making sure the 'svn_hash' temp file is created
in the same place the 'svn_delta_...' and 'git_blob_...' temp files are
(and then temp_release'd) so that it can be safely used inside the
close_file function without having its FileHandle end up in an SVN::Pool
that gets cleared.

Additionally the Git.pm cat_blob function creates a bidirectional pipe
FileHandle using the IPC::Open2::open2 function.  If that handle is
created too late, it also gets caught up in the SVN::Pool and incorrectly
closed by the SVN::Pool::clear call.  But this only seems to happen with
more recent versions of Perl and svn.

To avoid this problem we add an explicit call to _open_cat_blob_if_needed
before the first call to SVN::Pool->new_default to make sure the open2
handle does not end up in the SVN::Pool.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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