Make the memory-window conrol structures global

Up to now, the idea was that the user would do all the operations for
one repository in the same thread. Thus we could have the
memory-mapped window information thread-local and avoid any locking.

This is not practical in a few environments, such as Apple's GCD which
allocates threads arbitrarily or the .NET CLR, where the OS-level
thread can change at any moment.

Make the control structure global and protect it with a mutex so we
don't depend on the thread currently executing the code.

Handling of 100664 deprecated mode in tree entries

Clone

Minor fixes

Return -1 to stop the iteration instead of not-0

git_tree_walk callback return value semantic does not match documentation

Fix message prettify length check

If you want to be absolutely safe with git_message_prettify, you
can now pass a NULL pointer for the buffer and get back the number
of bytes that would be copied into the buffer.

This means that an error is a non-negative return code and a
success will be greater than zero from this function.

git_note_foreach: Fix documentation for notes_ref parameter

git_note_oid: Fix the documentation to reference parameters using the correct names

git_note_create: Copyediting on documentation for the oid parameter

Fix incorrect array size in example for git_config_get_mapped

Config file parser includes = in name if no space around it

Returning a negative cancels the walk, and returning a positive one
causes us to skip an entry, which was previously done by a negative
value.

This allows us to stay consistent with the rest of the functions that
take a callback and keeps the skipping functionality.

However, there should be a way to cancel the walk and another to skip
the entry.

Okay, this is probably cleaner and it is also less net change
from the original version

This makes the message prettify buffer length check accurate.

The config file parser was not working right if there was no
whitespace between the value name and the equals sign.  This
fixes that.

In the documentation for git_config_get_mapped, the sample mapping
array uses [3] but has 4 entries.  Fix by dropping the size entirely and
letting the compiler figure it out.

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.