Fix case insensitivity issues in git_status_file

Some doc improvements in transport.h

Introduce git_branch_tracking_name()

This makes tree iterators directly support case insensitivity by
using a secondary index that can be sorted by icase.  Also, this
fixes the ambiguity check in the git_status_file API to also be
case insensitive.  Lastly, this adds new test cases for case
insensitive range boundary checking for all types of iterators.

With this change, it should be possible to deprecate the spool
and sort iterator, but I haven't done that yet.

This adds a new external API git_tree_entry_cmp and a new internal
API git_tree_entry_icmp for sorting tree entries.  The case
insensitive one is internal only because general users should
never be seeing case-insensitively sorted trees.

This adds git_path_icmp to complement git_path_cmp.

git__bsearch and git__tsort did not pass a payload through to the
comparison function.  This makes it impossible to implement sorted
lists where the sort order depends on external data (e.g. building
a secondary sort order for the entries in a tree).  This commit
adds git__bsearch_r and git__tsort_r versions that pass a third
parameter to the cmp function of a user payload.

This adds a test that confirms that the working directory iterator
can actually correctly process ranges of files case insensitively
with proper sorting and proper boundaries.

This changes the iterator API so that flags can be passed in to
the constructor functions to control the ignore_case behavior.
At this point, the flags are not supported on tree iterators (i.e.
there is no functional change over the old API), but the API
changes are all made to accomodate this.

By the way, I went with a flags parameter because in the future
I have a couple of other ideas for iterator flags that will make
it easier to fix some diff/status/checkout bugs.

Returning GIT_EAMBIGUOUS from inside the status callback gets
overridden with GIT_EUSER.  `git_status_file` accounted for this
via the callback payload, but was allowing the error message to
be cleared.  Move the `giterr_set` call outside the callback to
where the EUSER case was being dealt with.

In preparation for further iterator changes, this cleans up a few
small things in the iterator API:

* removed the git_iterator_for_repo_index_range API
* made git_iterator_free not be inlined
* minor param name and test function name tweaks

checkout: Teach checkout to cope with orphaned Head

Some fixes for external template support

Before, it was identical to the default template making it difficult
to check, if the proper template was copied an external template
test.

been properly copied.

This is a minimal effort to test whether the template really has
been used when creating an repo with external templates.

This is an intermin solution. While this essentially disables the
--shared flag feature, previously external templates did not work
at all. This change fixes the previously corrected, and since
then failing, repo_init__extended_with_template() test.

The problem is now documented in the source code comments.

Fix #1236

The offset should be git_off_t, and we should check the return value
of the mutex lock function.

The indexer needs to call the packfile's free function so it takes care of
freeing the caches.

We still need to close the mwf descriptor manually so we can rename the
packfile into its final name on Windows.

Otherwise the template functionallity is not tested (as a TODO we
also shall test that the specified template really got copied).

Switch to specified branch during clone

Fixes #1232

Add a failing test for CRLF filters

This adds a git_buf_text_is_binary check to blobs before applying
filters when the blob data is being written to disk.

Match binary file check of core git in diff