git-svn: Make it incrementally faster by minimizing temp files

git-svn: Make it incrementally faster by minimizing temp files

Currently, git-svn would create a temp file on four occasions:
1. Reading a blob out of the object db
2. Creating a delta from svn
3. Hashing and writing a blob into the object db
4. Reading a blob out of the object db (in another place in code)

Any time git-svn did the above, it would dutifully create and then
delete said temp file.  Unfortunately, this means that between 2-4
temporary files are created/deleted per file 'add/modify'-ed in
svn (O(n)).  This causes significant overhead and helps the inode
counter to spin beautifully.

By its nature, git-svn is a serial beast.  Thus, reusing a temp file
does not pose significant problems.  "truncate and seek" takes much
less time than "unlink and create".  This patch centralizes the
tempfile creation and holds onto the tempfile until they are deleted
on exit.  This significantly reduces file overhead, now requiring
at most three (3) temp files per run (O(1)).

Signed-off-by: Marcus Griep <marcus@griep.us>
Acked-by: Eric Wong <normalperson@yhbt.net>