Our mkdir helper was failing is a parent directory was not
accessible even if the child directory could be created.
This changes the helper to keep trying child directories
even when the parent is unwritable.

Decode base64-encoded text into a git_buf

To make sure that items returned from pool allocations are aligned
on nice boundaries, this rounds up all pool allocation sizes to a
multiple of 8.  This adds a small amount of overhead to each item.

The rounding up could be made optional with an extra parameter to
the pool initialization that turned on rounding only for pools
where item alignment actually matters, but I think for the extra
code and complexity that would be involved, that it makes sense
just to burn a little bit of extra memory and enable this all the
time.

When checking out files, we're performing conversion into the user's
native line endings, but we only want to do it for files which have
consistent line endings. Refuse to perform the conversion for mixed-EOL
files.

The CRLF->LF filter is left as-is, as that conversion is considered to be
normalization by git and should force a conversion of the line endings.

When passed the LINK_FILES flag, the recursive copy will hardlink files
instead of copying them.

There are a number of tests that modify the global or system
search paths during the tests.  This adds a helper function to
make it easier to restore those paths and makes sure that they
are getting restored in a manner that preserves test isolation.

patch: emit deflated binary patches (optionally)