really reset walker with git_revwalk_reset

Update git_status_file and add ranged iterators

A few indexer consistency checks

This function fills in a git_buf with the common prefix of
an array of strings, but let's make that a little more clear.

Error out in finalize if there is junk after the packfile hash or we
couldn't process all the objects.

The goal of this work is to rewrite git_status_file to use the
same underlying code as git_status_foreach.

This is done in 3 phases:

1. Extend iterators to allow ranged iteration with start and
   end prefixes for the range of file names to be covered.
2. Improve diff so that when there is a pathspec and there is
   a common non-wildcard prefix of the pathspec, it will use
   ranged iterators to minimize excess iteration.
3. Rewrite git_status_file to call git_status_foreach_ext
   with a pathspec that covers just the one file being checked.

Since ranged iterators underlie the status & diff implementation,
this is actually fairly efficient.  The workdir iterator does
end up loading the contents of all the directories down to the
single file, which should ideally be avoided, but it is pretty
good.

From the description  of git_revwalk_reset in revwalk.h the function should
clear all pushed and hidden commits, and leave the walker in a blank state (just like at creation).
Apparently everything gets reseted appart of pushed commits (walk->one and walk->twos)

This fix should reset the walker properly.

Fix compilation warnings

Add git_note_foreach()

This reverts commit 1093e2de.

branch: cover with test that moving a non existing branch returns ENOTFOUND

branch: make git_branch_delete() return GIT_ENOTFOUND when the branch doesn't exist

See issue https://github.com/libgit2/libgit2/issues/680

Building a "shared object" (DLL) in Windows includes 2 steps:

- specify __declspec(dllexport)
  when building the library itself. MSVC will disallow itself from
  optimizing these symbols out and reference them in the PE's
  Exports-Table.
  Further, a static link library will be generated. This library
  contains the symbols which are exported via the declsepc above.
  The __declspec(dllexport) becomes part of the symbol-signature
  (like parameter types in C++ are 'mangled' into the symbol name,
  the export specifier is mingled with the name)

- specify __declspec(dllimport)
  when using the library. This again mingles the declspec into the
  name and declares the function / variable with external linkage.

cmake automatically adds -Dgit2_EXPORTS to the compiler arguments
when compiling the libgit2 project.
The 'git2' is the name specified via PROJECT() in CMakeLists.txt.

Add git_blob_create_fromdisk()

Fix build/runtime issues on Solaris

Attribute cache buster

Use -fvisibility=hidden in GCC builds

mergebase: enhance test code coverage

iterator: prevent git_iterator_free() from segfaulting when being passed a NULL iterator

Since we now rely on it (at least under Solaris), I figured we probably
want to make sure it's accurate. The new test makes sure that creating a
file with a name of length FILENAME_MAX+1 fails.

This function will create blobs in the object database from files anywhere on the filesystem. This can be run against bare and non-bare repositories.

On Solaris, struct dirent is defined differently than Linux. The field
containing the path name is of size 0, rather than NAME_MAX. So, we need to
use a properly sized buffer on Solaris to avoid a stack overflow.

Also fix some DIR* leaks on cleanup.

Improve repo initialization to be more like git

object: make git_object_lookup() return GIT_ENOTFOUND - The sequel

object: make git_object_lookup() return GIT_ENOTFOUND when searching for an existing object by specifying an incorrect type

This fix complements cb0ce16b and cover the following additional use cases

 - retrieving an object which has been previously searched, found and cached
 - retrieving an object through an non ambiguous abbreviated id