If we remove a reference while we're iterating through the packed refs,
the position in the iterator will be off.

This adds another assertion to ensure that the reference name inside
the git_reference struct returned by `git_branch_create` is returned as
precomposed if `core.precomposeunicode` is enabled.

This tests that decomposed branch names are correctly precomposed when
passed to `git_branch_move` and `core.precomposeunicode` is enabled.

Only on a filesystem that is composed/decomposed insensitive,
should be testing that a branch can be looked up by the opposite
form and still work correctly.

When using Iconv to convert unicode data and iconv doesn't like
the source data (because it thinks that it's not actual UTF-8),
instead of stopping the operation, just use the unconverted data.
This will generally do the right thing on the filesystem, since
that is the source of the non-UTF-8 path data anyhow.

This adds some tests for creating and looking up branches with
messy Unicode names.  Also, this takes the helper function that
was previously internal to `git_repository_init` and makes it
into `git_path_does_fs_decompose_unicode` which is a useful in
tests to understand what the expected results should be.

Trying to find other issues where tests may not clean up quite
properly when they are through...

git_branch_t is an enum so requesting GIT_BRANCH_LOCAL | GIT_BRANCH_REMOTE is not possible as it is not a member of the enum (at least VS2013 C++ complains about it).

This fixes a regression introduced in commit a667ca8298193b3103c1dbdcb1f6c527e6e99eb2 (PR #1946).

Signed-off-by: Sven Strickroth <email@cs-ware.de>

On bare by default, or when core.logallrefupdates is false, we must not
write the reflog.

If the caller wants to update a ref to point to the same target as it
currently has, we should return early and avoid writing to the reflog.