Adds a SelectByName method to the archive and scanner which only require
the filename as input, and can thus be run before calling lstat on the
file. Can speed up scanning significantly if a lot of filename excludes
are used.
The option is named --exclude-if-present and accepts a parameter
filename[:content]. Directories are excluded and their contents is not
backed up if they contain a file with the specified name and,
optionally, that starts with the specified content. The tagfile itself
is never excluded.
There is also a shortcut --exclude-caches that works in the same way as
the likewise-named option of tar(1): Directories are recognized as cache
if they contain a file named "CACHEDIR.TAG.
Closes #317.
