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

In private function 'load_blob'.

Fixed compilation on Windows when using libssh2.

Reorder revert test variable decls

Oh, MSVC.

Bare naked merge and rebase

Include git2/revert.h in git2.h

Updates to cancellation logic during download and indexing of packfile.

Revert support for a single commit

Add GIT_DIFF_FIND_REMOVE_UNMODIFIED flag and fix copy detection bug

When doing copy detection, it is often necessary to include
UNMODIFIED records in the git_diff so they are available as source
records for GIT_DIFF_FIND_COPIES_FROM_UNMODIFIED.  Yet in the final
diff, often you will not want to have these UNMODIFIED records.
This adds a flag which marks these UNMODIFIED records for deletion
from the diff list so they will be removed after the rename detect
phase is over.

When FIND_COPIES is used in combination with BREAK_REWRITES for
rename detection, there was a bug where the split MODIFIED delta
was only used as a target for RENAME records and not for COPIED
records.  This fixes that, converting the split into a pair of
DELETED and COPIED deltas when that circumstance arises.

Improve iconv finding for cmake

* add FindIconv helper for CMake iconv detection
* only default using iconv to ON for MacOS
* update pkg-config generation to include iconv dependency better

fix typos in docs

posix: Solaris doesn't have strnlen either