transport: let the progress output return an error

There are any number of issues that can come up in the progress
callback, and we should let the user cancel at that point as well.

RFC: Proposed reworking of diff APIs

It seems that to implement these options, we just have to pass
the appropriate flags through to the libxdiff code taken from
core git.  So let's do it (and add a test).

Instead of having functions with so very many parameters to pass
hunk and line data, this takes the existing git_diff_hunk struct
and extends it with more hunk data, plus adds a git_diff_line.
Those structs are used to pass back hunk and line data instead of
the old APIs that took tons of parameters.

Some work that was previously only being done for git_diff_patch
creation (scanning the diff content for exact line counts) is now
done for all callbacks, but the performance difference should not
be noticable.

Allowed credential types should be a bitfield

While the base git_diff_delta structure always contains two files,
when we introduce conflict data, it will be helpful to have an
indicator when an additional file is involved.

license clarification + formatting all over

fixed typo `can can` -> `can`

added the second change suggested by @nulltoken (with two extra words :) )

There are commercial open source applications, hence proprietary seems to be a better term.

Also:
 - Trimmed header `=`'s to match text length
 - made libgit2 stand out everywhere
 - pretty printing links
 - irc link for those with the correct setup

Checkout merge

It seemed exceptionally silly to have a split there
where no split needed to be.

Move conflict handling into two steps: load the conflicts and
then apply the conflicts.  This is more compatible with the
existing checkout implementation and makes progress reporting
more sane.

If a D/F conflict or rename 2->1 conflict occurs,
we write the file sides as filename~branchname.  If
a file with that name already exists in the working
directory, write as filename~branchname_0 instead.
(Incrementing 0 until a unique filename is found.)

rename conflict tests for checkout conflicts, don't suffix filenames
when checking out with USE_OURS or USE_THEIRS

Prevent checkout tree when unresolved changes exist (unless FORCE flag
is specified).  Clear NAME table when checking out, to avoid
checkout_conflicts from attempting to manipulate it.  Ensure that NAME
is also cleared at reset.

This lays groundwork for separating formatting options from diff
creation options.  This groups the formatting flags separately
from the diff list creation flags and reorders the options.  This
also tweaks some APIs to further separate code that uses patches
from code that just looks at git_diffs.

This makes no functional change to diff but renames a couple of
the objects and splits the new git_patch (formerly git_diff_patch)
into a new header file.

Provide the user with a more useful error code when a looking up a reference which name points to a namepace

This lets the reference code return not-found when the user asks to
look up a reference when in fact they pass a namespace.

Fix a typo in the rpm build instructions.

The iconv init was accidentally clearing the default error state
during reference normalization. This resets so that normalization
errors will be detected correctly.

Add support for core.precomposeunicode on Mac

Before these changes, looking up a reference would return the
same precomposed or decomposed form of the reference name that
was used to look it up, so on MacOS which ignores the difference
between the two, a single reference could be looked up either way
and git_reference_name would return the form of the name that was
used to look it up!  This change makes lookup always return the
precomposed name if core.precomposeunicode is set regardless of
which version was used to look it up.  The reference iterator was
already returning the precomposed form from earlier work.

This also updates the CMakeLists.txt rules for enabling iconv
usage because the clar tests for this code were actually not being
activated properly with the old version.

Finally, this moves git_repository_reset_filesystem from include/
git2/repository.h to include/git2/sys/repository.h since it is not
really a function that normal library users should have to think
about very often.

WinHTTP: set Accept header for POSTs

A couple of tests were actually dealing incorrectly with case
sensitivity issues on Linux because they were relying on having
core.ignorecase set to true.  Now that the fixture initialization
sets the case sensitivity to be accurate for the platform, it
exposed bugs in these tests.