It is okay to not have a .gitconfig file

Fix win32 lstat

Opening a repo is generating an error if you don't have a
.gitconfig file in your home directory, but that should be
legal.

VS-Build: Append C_FLAGS to the end of the already given ones.

My other PR revealed, that the /W4 parameter, we give to MSVC is ignored
because cmake set CMAKE_C_FLAGS already to /W3 and we overwrite it.

The command line tools gave me a D9025 warning for this on every file and
looking into the project properties page on MSVC 2008 tells, that it has
the warning level set to /W3.

However, the warnings introduced by /W4 are far to useless for having
them enabled. So just disable them.

Don't use precompiled headers for command-line based VC builds.

The reason, why libgit2 currently cannot support compiling via the command-line
tools cl/nmake from WinSDK and/or Microsoft Visual Studio, seems to be a
missing dependency on the generated precompiled header file.

The Visual Studio IDE automatically inserts this dependency when it sees the
right combination of "/Y" parameters.

This patch allows to compile using command line tools by disabling precompiled
headers for NON-IDE builds.

Filename validation

This prevents the index api from calling write_tree() with a
bogus tree.

As per core git.

Indexer: Avoid a possible double-deletion in error case

Itsy bitsy tiny winy minor api related fixes

Passing 0 as the index now retrieves the most recent entry instead
of the oldest one.

odb: recursively load alternates

The maximum depth is 5, like in git

Win32 fixes for diff/checkout/reset

Add option to ignore file mode in diffs

Diff API signatures