Clean up some error handling and change callback error behavior

The size_t is 32-bit already, so it overflows before going into the
function. The `-1` test should handle this gracefully in both cases
anyway.

Ok, scrap the previous commit. This is the right overflow check that
takes care of 64 bit overflow **and** 32-bit overflow, which needs to be
considered because the pool malloc can only allocate 32-bit elements in
one go.

Note that `git_pool_strdup` cannot really return any error codes,
 because the pool doesn't set errors on OOM.

 The only place where `giterr_set_oom` is called is in
 `git_pool_strndup`, in a conditional check that is always optimized
 away. `n + 1` cannot be zero if `n` is unsigned because the compiler
 doesn't take wraparound into account.

 This check has been removed altogether because `size_t` is not
 particularly going to overflow.

Validate struct versions in merge, revert

This wasn't being tested and since it has a callback, I fixed it
even though the return value of this callback is not treated like
any of the other callbacks in the API.

This renames git_vector_free_all to the better git_vector_free_deep
and also contains a couple of memory leak fixes based on valgrind
checks.  The fixes are specifically: failure to free global dir
path variables when not compiled with threading on and failure to
free filters from the filter registry that had not be initialized
fully.

This covers diff print, push, and ref foreach.  This also has a
fix for a small memory leak in the push tests.

This adds tests that try canceling an indexer operation from
within the progress callback.

After writing the tests, I wanted to run this under valgrind and
had a number of errors in that situation because mmap wasn't
working.  I added a CMake option to force emulation of mmap and
consolidated the Amiga-specific code into that new place (so we
don't actually need separate Amiga code now, just have to turn on
-DNO_MMAP).

Additionally, I made the indexer code propagate error codes more
reliably than it used to.

This time actually checking return values in diff notify tests and
actually testing callbacks for the index all-all/update-all/etc
functions.

Clone callbacks can return non-zero values to cancel the clone.
This adds some tests to verify that this actually works and updates
the documentation to be clearer that this can happen and that the
return value will be propagated back by the clone function.

The checkout notify callback behavior on non-zero return values
was not being tested.  This adds tests, fixes a bug with positive
values, and clarifies the documentation to make it clear that the
checkout can be canceled via this mechanism.

The callback to supply data chunks could return a negative value
to stop creation of the blob, but we were neither using GIT_EUSER
nor propagating the return value.  This makes things use the new
behavior of returning the negative value back to the user.

Okay, I've decided I like the readability of this style much
better so I used it everywhere.

I find this easier to read...

This changes the behavior of callbacks so that the callback error
code is not converted into GIT_EUSER and instead we propagate the
return value through to the caller.  Instead of using the
giterr_capture and giterr_restore functions, we now rely on all
functions to pass back the return value from a callback.

To avoid having a return value with no error message, the user
can call the public giterr_set_str or some such function to set
an error message.  There is a new helper 'giterr_set_callback'
that functions can invoke after making a callback which ensures
that some error message was set in case the callback did not set
one.

In places where the sign of the callback return value is
meaningful (e.g. positive to skip, negative to abort), only the
negative values are returned back to the caller, obviously, since
the other values allow for continuing the loop.

The hardest parts of this were in the checkout code where positive
return values were overloaded as meaningful values for checkout.
I fixed this by adding an output parameter to many of the internal
checkout functions and removing the overload.  This added some
code, but it is probably a better implementation.

There is some funkiness in the network code where user provided
callbacks could be returning a positive or a negative value and
we want to rely on that to cancel the loop.  There are still a
couple places where an user error might get turned into GIT_EUSER
there, I think, though none exercised by the tests.

There are a lot of places that we call git__free on each item in
a vector and then call git_vector_free on the vector itself.  This
just wraps that up into one convenient helper function.

This continues auditing all the places where GIT_EUSER is being
returned and making sure to clear any existing error using the
new giterr_user_cancel helper.  As a result, places that relied
on intercepting GIT_EUSER but having the old error preserved also
needed to be cleaned up to correctly stash and then retrieve the
actual error.

Additionally, as I encountered places where error codes were not
being propagated correctly, I tried to fix them up.  A number of
those fixes are included in the this commit as well.

This adds giterr_user_cancel to return GIT_EUSER and clear any
error message that is sitting around.  As a result of using that
in places, we need to be more thorough with capturing errors that
happen inside a callback when used internally.  To help with that,
this also adds giterr_capture and giterr_restore so that when we
internally use a foreach-type function that clears errors and
converts them to GIT_EUSER, it is easier to restore not just the
return value, but the actual error message text.

This adds `git_config__lookup_entry` which will look up a key in
a config and return either the entry or NULL if the key was not
present.  Optionally, it can either suppress all errors or can
return them (although not finding the key is not an error for this
function).  Unlike other accessors, this does not normalize the
config key string, so it must only be used when the key is known
to be in normalized form (i.e. all lower-case before the first dot
and after the last dot, with no invalid characters).

This also adds three high-level helper functions to look up config
values with no errors and a fallback value.  The three functions
are for string, bool, and int values, and will resort to the
fallback value for any error that arises.  They are:

* `git_config__get_string_force`
* `git_config__get_bool_force`
* `git_config__get_int_force`

None of them normalize the config `key` either, so they can only
be used for internal cases where the key is known to be in normal
format.

Rename detection using diff.renames

Clean up warnings

commit: Fix potential segfault

Dereferencing commit pointer before asserting

Fixed left shift size of int.

Simply switched the ordering of the checks in the for loop where this left
shift was being made.

blame.c: Remove unnecessary error-check and goto