Fix MSVC compilation errors

Branch foreach

Cleanup

fix crash issue on mingw caused by variable argument list type promotion.

1. compile warning:

D:\libgit2.git\src\win32\posix_w32.c: In function 'p_open':
D:\libgit2.git\src\win32\posix_w32.c:235:10: warning: 'mode_t' is promoted to 'int' when passed through '...' [enabled by default]
D:\libgit2.git\src\win32\posix_w32.c:235:10: note: (so you should pass 'int' not 'mode_t' to 'va_arg')
D:\libgit2.git\src\win32\posix_w32.c:235:10: note: if this code is reached, the program will abort

2. test crash.

3. the above two issues are same root cause. please see http://www.eskimo.com/~scs/cclass/int/sx11c.html

The second call to assert_config_entry_on_init_bytype is cleaned up by
the main cleanup function, but that overwrites the first _repo. Make
sure that one doesn't leak.

The call to repo_init_reinit already takes care of opening the
repository and giving us a git_repository object to give to the
caller. There is no need to call git_repository_open again.

Fix Makefile.emebed for mingw32

Don't need unix/*.c for windows, don't need wildcard for regex.c

Revparse fixup

otherwise we can't compile the native parts of the rugged gem on Windows

When the creation of one iterator fails, we need to free the prefix
and possibly one of the iterators. Make sure we do so.

If we find several objects with the same prefix, we need to free the
memory where we stored the earlier object. Keep track of the raw.data
pointer across read_prefix calls and free it if we find another
object.

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()