filter: add docs for streaming filters

These functions are available on the public API but don't have any
documentation, so they don't appear on the API reference. Fix that.

iterator_walk: cast away constness for free

filter::stream: free the filter sanely

Don't use the filter's free callback to free the actual data structure
holding the filter, as we may not always actually initialize it (the
test may be skipped).

Correctly delimit the keys for submodule lookup

The regex we use to look at the gitmodules file does not correctly
delimit the name of submodule which we want to look up and puts '.*'
straight after the name, maching on any submodule which has the seeked
submodule as a prefix of its name.

Add the missing '\.' in the regex so we want a full stop to exist both
before and after the submodule name.

When two submodules are fairly similar, we may end up loading the wrong
one.

Fix 8.3 filename tests failure when 8.3 is disabled

The function was removed, but its declaration and changelog entry about
its removal were forgotten.

The comment in the test doesn't make any sense as the function doesn't
exist anymore, so get rid of it as well.

Fix #3093 - remove declaration of unused function git_fetch__download…

Function was added in commit 2c982daa on October 5, 2011,
and removed in commit 41fb1ca0 on October 29, 2012.
Given the length of time it's gone unused, it's safe to remove now.

More warnings

http: fixed leak when asking for credentials again

t->cred might have been allocated the previous time and needs to be
freed before asking caller for credentials again.

memory leak refspec.c

Remove some warnings

Submodule and http fixes

When the server rejects an authentication request, ask the caller for
the credentials again, instead of giving up on the first try.

We allow looking up a submodule by path, but we lost the path
normalisation during the recent changes. Bring it back.

Remove some of the logic that was left-over from the time we had a cache
of submodules, plugging a leak of the submodule object in certain cases.