Add helper API for diff line stats from patch

This adds a `git_diff_patch_line_stats()` API that gets the total
number of adds, deletes, and context lines in a patch.  This will
make it a little easier to emulate `git diff --stat` and the like.

Right now, this relies on generating the `git_diff_patch` object,
which is a pretty heavyweight way to get stat information.  At
some future point, it would probably be nice to be able to get
this information without allocating the entire `git_diff_patch`,
but that's a much larger project.

Stricter config entry name validation

Fix p_realpath on OpenBSD

Vector improvements and their fallout

This is @nulltoken's work to test various invalid config section
and key names and make sure we are validating properly.

This is a new implementation of core git's config key checking
rules that prevents non-alphanumeric characters (and '-') for
the top-level section and key names inside of config files.

This also validates the target section name when renaming
sections.

OpenBSD's realpath(3) doesn't require the last part of the path to
exist. Override p_realpath in this OS to bring it in line with the
library's assumptions.

Added git_treebuilder_entrycount

Conflicts:
	src/tree.c

- Fix stack corruption introduced in 9bccf33c due to passing pointer to
local variable _cred_acquire_called.
- Fix strcmp in do_verify_push_status when expected or actual push_status
  is NULL

Fix local repository clone failure

avoid use object which is already free

Signed-off-by: Frank Li <lznuaa@gmail.com>

Use cl_assert_equal_s() instead of strcmp().

Improve valgrind suppressions

config: support trailing backslashes

clone: Prevent segfault upon faulted remote creation

Check whether the backslash at the end of the line is being escaped or
not so as not to consider it a continuation marker when it's e.g. a
Windows-style path.

Replaced all cl_assert(!strcmp()) or semantically equivalent forms
by cl_assert_equal_s().

Add git_branch_name()

This is a convenience function to get the branch name of a given
ref. The returned branch name is compatible with the name that can
be supplied e.g. to git_branch_lookup(). That is, the prefixes
"refs/heads" or "refs/remotes" are omitted.

Also added a new test for testing the new function.

The index is empty; repeated tree entries cannot collide.

cc github/gitrpc#83