Profiling `git annex find --in web` found that a single decodeFilePath in
gitAnnexIndex uses 1.4% of time.

The call path is getRef to withIndex to gitAnnexIndex.

The FilePath is put into the environment. Switching to use RawFilePath
environment when running processes would be a bit involved, because
System.Process does not support it and would need to be modified/forked.
(The rawfilepath package did it, but unfortunately also changed its API in
other ways.)

However... git-annex starts up a single, long-running git cat-file
process. The only reason it needs to get gitAnnexIndex after that is
running is to select the git process that is using the right index file.

So, one way would be to make withIndexFile less generic,
eg a withAnnexIndexFile that does not need the filename to be calculated
each time.

Or, keep withIndexFile generic but change Annex.cachedgitenv to contain
ByteStrings, and convert to FilePath only when that environment is used to
start a new process. (This seems like it would be a little slower than
the other alternative, since constructing a RawFilePath is also not
entirely without cost, although significantly faster.)

--[[Joey]] 

> [[done]], and benchmarking shows at least 1.75% speedup --[[Joey]]
