And use 0 for GIT_FILEMODE_UNREADABLE.

When thees is an unreadable folder, we should still be able
to enumerate status.

Pass unconverted Unicode path data when iconv doesn't like it

Be more careful with user-supplied buffers

Some improvements to the cert checking

Configuration snapshotting

Add filter options and ALLOW_UNSAFE

It seems that with the various recent changes to reference updating
and reflog writing, that the thread safety of refdb updates has
been reduced (either that or it was never thread safe and the
window for error has increased).  Either way, this test is now
sometimes segfaulting which is no good, so let's disable the test
for now.  We don't really make any public promises about thread
safety for this type of operation, so I think this is acceptable,
at least in the short term.

Only on a filesystem that is composed/decomposed insensitive,
should be testing that a branch can be looked up by the opposite
form and still work correctly.

One of the test helpers provides a quick way for looking up a
boolean key.  But if the key way missing completely, the check
would actually raise an error.  Given the way we use this helper,
if the key is missing, this should just return false, I think.

indexer: avoid memory moves

When using Iconv to convert unicode data and iconv doesn't like
the source data (because it thinks that it's not actual UTF-8),
instead of stopping the operation, just use the unconverted data.
This will generally do the right thing on the filesystem, since
that is the source of the non-UTF-8 path data anyhow.

This adds some tests for creating and looking up branches with
messy Unicode names.  Also, this takes the helper function that
was previously internal to `git_repository_init` and makes it
into `git_path_does_fs_decompose_unicode` which is a useful in
tests to understand what the expected results should be.

Our vector does a move of the rest of the array when we remove an
item. Doing this repeatedly can be expensive, and we do this a lot in
the indexer. Instead, set the value to NULL and skip those entries.

perf reported around 30% of `index-pack` time was going into
memmove. With this change, that goes away and we spent most of the time
hashing and inflating data.

iconv debugging aids

This is quite close to running "git for-each-ref" except:

  1. It does not take any formatting or selection options at
     all.

  2. The output is not sorted.

I wrote it to look at debugging some issues with ref
iteration, but there's no reason it can't live on as an
example command.

The cmake module we provide is in the file FindIconv.cmake,
so we must match the case correctly. It happens to work in
practice because we only turn on ICONV on Darwin, and people
generally have case-insensitive filesystems there.

Note that we only need to update the package name here. The
package itself still sets the all-uppercase ICONV_FOUND
flag, so we continue to use uppercase in the rest of cmake.

This adds in missing calls to `git_buf_sanitize` and fixes a
number of places where `git_buf` APIs could inadvertently write
NUL terminator bytes into invalid buffers.  This also changes the
behavior of `git_buf_sanitize` to NUL terminate a buffer if it can
and of `git_buf_shorten` to do nothing if it can.

Adds tests of filtering code with zeroed (i.e. unsanitized) buffer
which was previously triggering a segfault.

Accessing the repository's config and immediately taking a snapshot of
it is a common operation, so let's provide a convenience function for
it.

Fix the issues in git_shutdown

Diff and status do not want core.safecrlf to actually raise an
error regardless of the setting, so this extends the filter API
with an additional options flags parameter and adds a flag so that
filters can be applied with GIT_FILTER_OPT_ALLOW_UNSAFE, indicating
that unsafe filter application should be downgraded from a failure
to a warning.

odb: ignore files in the objects dir