fscache: load directories only once

If multiple threads access a directory that is not yet in the cache, the
directory will be loaded by each thread. Only one of the results is added
to the cache, all others are leaked. This wastes performance and memory.

On cache miss, add a future object to the cache to indicate that the
directory is currently being loaded. Subsequent threads register themselves
with the future object and wait. When the first thread has loaded the
directory, it replaces the future object with the result and notifies
waiting threads.

Signed-off-by: Karsten Blees <blees@dcon.de>
Merge pull request #205 from kblees/kb/fscache-load-directories-once

fscache: load directories only once
symlinks: remove PATH_MAX limitation

'git checkout' fails if a directory is longer than PATH_MAX, because the
lstat_cache in symlinks.c checks if the leading directory exists using
PATH_MAX-bounded string operations.

Remove the limitation by using strbuf instead.

Signed-off-by: Karsten Blees <blees@dcon.de>
dir: remove PATH_MAX limitation

'git status' segfaults if a directory is longer than PATH_MAX, because
processing .gitignore files in prep_exclude() writes past the end of a
PATH_MAX-bounded buffer.

Remove the limitation by using strbuf instead.

Note: this fix just 'abuses' strbuf as string allocator, len is always 0.
prep_exclude() can probably be simplified using more strbuf APIs.

Signed-off-by: Karsten Blees <blees@dcon.de>
Win32: fix 'lstat("dir/")' with long paths

Use a suffciently large buffer to strip the trailing slash.

Signed-off-by: Karsten Blees <blees@dcon.de>