Introduce GIT_DIFF_FIND_BY_CONFIG
Implement GIT_DIFF_FIND_BY_CONFIG
Don't use weird return codes
blame.c: Remove unnecessary error-check and goto

In private function 'load_blob'.
Merge pull request #1989 from palistov/blame-cleanup

blame.c: Remove unnecessary error-check and goto
Clarify default value and behavior
Don't clobber whitespace settings
Clarify docs
Fixed left shift size of int.

Simply switched the ordering of the checks in the for loop where this left
shift was being made.
Merge pull request #1993 from jaredlwong/development

Fixed left shift size of int.
commit: Fix potential segfault in git_commit_message

Dereferencing commit pointer before asserting
Merge pull request #1994 from palistov/commit-cleanup

commit: Fix potential segfault
Clean up warnings
Merge pull request #1996 from ethomson/warnings

Clean up warnings
Check version earlier
Merge pull request #1985 from libgit2/diff-rename-config

Rename detection using diff.renames
Add config read fns with controlled error behavior

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.
Improve GIT_EUSER handling

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.
Further EUSER and error propagation fixes

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.
Add git_vector_free_all

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.