tree cache: loosen negative entry count check

While C Git has been writing entry count -1 (ie. never other negative
numbers) as invalid since day 1, it accepts all negative entry counts
as invalid. JGit follows the same rule. libgit2 should also follow, or
the index that works with C Git or JGit may someday be rejected by
libgit2.

Other reimplementations like dulwich and grit have not bothered with
parsing or writing tree cache.

ignore mkmf.log

no build artifacts
no parents

More iterator cleanup

The `git_iterator_reset` command has not been working in all cases
particularly when there is a start and end range.  This fixes it
and adds tests for it, and also extends it with the ability to
update the start/end range strings when an iterator is reset.

This removes the need to explicitly pass the repo into iterators
where the repo is implied by the other parameters.  This moves
the repo to be owned by the parent struct.  Also, this has some
iterator related updates to the internal diff API to lay the
groundwork for checkout improvements.

Add a payload param to git_cred_acquire_cb

Fixes #1128.

indexer: move the temporary buffers into the indexer object

Fix mwindow mutex initialization and error checking

This could also use PTHREAD_MUTEX_INITIALIZER, but a dynamic initializer seems like a more portable concept, and we won't need another #define on top of git_mutex_init()

Storing 4kB or 8kB in the stack is not very gentle. As this part has
to be linear, put the buffer into the indexer object so we allocate it
once in the heap.

Indexer speedup with large objects

tree: relax the filemode parser

There are many different broken filemodes in the wild so we need to
protect against them and give something useful up the chain. Don't
fail when reading a tree from the ODB but normalize the mode as best
we can.

As 664 is no longer a mode that we consider to be valid and gets
normalized to 644, we can stop accepting it in the treebuilder. The
library won't expose it to the user, so any invalid modes are a bug.

Fix diff header comments and missing const

Based on the recent work to wrap diff in objective-git, this
includes a fix for a missing const and a number of clarifications
of the documentation.

Version info for public structs