Export git_attr_value

Commit 0c9eacf3 introduced the function
git_attr_value and switched the GIT_ATTR_* macros to use it, but
attempting to use that function leads to a linker error (undefined
reference to `git_attr_value').  Export git_attr_value so programs can
actually call it.

Parse ref oids without trailing newline

There is a little cleanup necessary from PR #843.  Since the
new callbacks return `GIT_EUSER` we have to be a little careful
about return values when they are used internally to the library.

Also, callbacks should be checked for non-zero return values,
not just less than zero.

Conflicts:
	src/notes.c
	src/transports/git.c
	src/transports/http.c
	src/transports/local.c
	tests-clar/odb/foreach.c

Otherwise we get an incomplete type error, since git_remote_callbacks
isn't declared yet.

Missed this one, ironically enough.

This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:

* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
  (i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
  the library and `giterr_last()` will return NULL if called.

This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.

Conflicts:
	src/netops.c
	src/netops.h
	src/oid.c

attr: Do not export variables externally

repository: add a getter and remove function for git's prepared message

Fixes #824

Exporting variables in a dynamic library is a PITA. Let's keep
these values internally and wrap them through a helper method.

This doesn't break the external API. @arrbee, aren't you glad I turned
the `GIT_ATTR_` macros into function macros? ✨

Add function to query for compile time settings.

The 'git revert/cherry-pick/merge -n' commands leave .git/MERGE_MSG
behind so that git-commit can find it. As we don't yet support these
operations, users who are shelling out to let git perform these
operations haven't had a convenient way to get this message.

These functions allow the user to retrieve the message and remove it
when she's created the commit.

git_config_find_* does not set a git error

travis: be more idiomatic with the environment

odb: allow creating an ODB backend from a packfile index

Unify the transport code

SSL isn't the only way that a transport can be encrypted. The new name
will make it easier to merge the SSH support.

Using the callbacks makes it clearer and reduces the amount of #ifdefs
and ifs and we need in the code.

Instad of each transport having its own function and logic to get to
its refs, store them directly in transport.

Leverage the new gitno_buffer to make the parsing and storing of the
refs use common code and get rid of the git_protocol struct.

This allows us to add capabilitites to both at the same time, keeps
them in sync and removes a lot of code.

gitno_buffer now uses a callback to fill its buffer, allowing us to
use the same interface for git and http (which uses callbacks).