This fixes git_index_add and git_index_append to behave more like
core git, preserving old filemode data in the index when adding
and/or appending with core.filemode = false.

This also has placeholder support for core.symlinks and
core.ignorecase, but those flags are not implemented (well,
symlinks has partial support for preserving mode information in
the same way that git does, but it isn't tested).

Fix potential segfault in revparse.

Quotes inside config values don't survive serialization/deserialization

Expose git_merge_base_many()

Conflicts:
	src/notes.c

Errors were due to not including winsock2 early enough.

Precompile headers for MSVC.

When a configuration option is set, we didn't check to see whether
there was any escaping needed. Escape the available characters so we
can unescape them correctly when we read them.

Fix issue #763

make git_repository_init() value the core.logallrefupdates config entry 

This function has been available for some time, but never in a
header. Expose it so we can use it from outside the library.

git_status_file returns GIT_ENOTFOUND for files in "foo/" when .gitignore contains "foo bar.txt"

Fix filemode comparison in diffs

Missed a place that used this and missed git__isalpha

When checking for a drive letter on windows, instead of using
isalpha(), it is better to just check for a..z and A..Z, I think,
particularly because the MS isalpha implementation appears to
assert when given an 0xFF byte.

There are three actual changes in this commit:

1. When the trailing newline of a file is removed in a diff, the
   change will now be reported with `GIT_DIFF_LINE_DEL_EOFNL` passed
   to the callback.  Previously, the `ADD_EOFNL` constant was given
   which was just an error in my understanding of when the various
   circumstances arose.  `GIT_DIFF_LINE_ADD_EOFNL` is deprecated and
   should never be generated.  A new newline is simply an `ADD`.
2. Rewrote the `diff_delta__merge_like_cgit` function that contains
   the core logic of the `git_diff_merge` implementation.  The new
   version doesn't actually have significantly different behavior,
   but the logic should be much more obvious, I think.
3. Fixed a bug in `git_diff_merge` where it freed a string pool
   while some of the string data was still in use.  This led to
   `git_diff_print_patch` accessing memory that had been freed.

The rest of this commit contains improved documentation in `diff.h`
to make the behavior and the equivalencies with core git clearer,
and a bunch of new tests to cover the various cases, oh and a minor
simplification of `examples/diff.c`.

File modes were both not being ignored properly on platforms
where they should be ignored, nor be diffed consistently on
platforms where they are supported.

This change adds a number of diff and status filemode change
tests.  This also makes sure that filemode-only changes are
included in the diff output when they occur and that filemode
changes are ignored successfully when core.filemode is false.

There is no code that automatically toggles core.filemode
based on the capabilities of the current platform, so the user
still needs to be careful in their .git/config file.

Tests: wrap 'getenv' and friends for Win32 tests.

 - Do not create new levels of fanout when creating notes from libgit2
 - Insert a note in an existing matching fanout
 - Remove a note from an existing fanout
 - Cleanup git_note_read, git_note_remove, git_note_foreach, git_note_create methods in order use tree structures instead of tree_oids