squash! Verify memoized files can be reloaded before using them

As per

#246 (comment)

replace the commit message by:

git-svn: do not reuse caches memoized for a different architecture

Reusing cached data speeds up git-svn by quite a fair bit. However, if
the YAML module is unavailable, the caches are written to disk in an
architecture-dependent manner. That leads to problems when upgrading,
say, from 32-bit to 64-bit Git for Windows.

Let's just try to read those caches back if we detect the absence of the
YAML module and the presence of the file, and delete the file if it
could not be read back correctly.

Note that the only way to catch the error when the memoized cache could
not be read back is to put the call inside an `eval { ... }` block
because it would die otherwise; the `eval` block should also return `1`
in case of success explicitly since the function reading back the cached
data does not return an appropriate value to test for success.

This fixes #233.

[jes: fixed the commit message, made the sign-off explicit]

Signed-off-by: Gavin Lambert <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho