Make index add/append support core.filemode flag

message: Expose git_message_prettify()

oid_for_tree_path may not always find the path in the tree, in which
case we need to return an error. The current code doesn't do this and
results in undefined behavior.

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.

git_commit() and git_tag() no longer prettify the
message by default. This has to be taken care of
by the caller.

This has the nice side effect of putting the
caller in position to actually choose to strip
the comments or not.

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`.