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.

Add git_reset()

Rev parse

Currently supports Soft and Mixed modes.

Add the ability to create blob given a provider of chunks of bytes

Fix compilation warning and failing test

git_status_file returns GIT_ENOTFOUND for "域名检测工具.exe" (and similarly named files) 

git_status_file would always return GIT_ENOTFOUND for these files.

The underlying bug was that git__strcmp_cb, which is used by
git_path_with_stat_cmp to sort entries in the working directory,
compares strings based on unsigned chars (this is confirmed by the
strcmp(3) manpage), while git__prefixcmp, which is used by
workdir_iterator__entry_cmp to search for a path in the working
directory, compares strings based on char. So the sort puts this path at
the end of the list, while the search expects it to be at the beginning.

The fix was simply to make git__prefixcmp compare using unsigned chars,
just like strcmp(3). The rest of the change is just adding/updating
tests.

Converted an internal utility to return an oid, 
rather than a tree entry (whose lifetime is tied
to the parent tree, which was freed before 
returning).

This test is intended to verify that 64-bit
machines can handle parsing dates in 2039 and
beyond, and fails on 32-bit machines. It is now
omitted when run on a 32-bit machine to eliminate
an expected failure.