Add ODB API to check for existence by prefix and object id shortener

refdb: catch a directory disappearing

ODB writing fails gracefully when unsupported

If a directory disappears between the time we look up the entries of its
parent and the time when we go to look at it, we should ignore the error
and move forward.

This fixes #2046.

If no ODB backends support writing, we should fail gracefully.

examples/diff: Add minimal, patience diff options.

Fixed missing error check on call to git_remote_download

Fixed missing error check on call to git_remote_download in git_remote_fetch. Moved error check to statement following git_remote_disconnect so that the disconnect happens regardless of the result of the download call.

- Add minimal, patience diff options to diff example. libgit2
  `diff_xdiff.git_xdiff_init` already supports these flags, so
  no additional change is necessary.
- Remove minimal and patience flag addition from project list.

This finds a short id string that will unambiguously select the
given object, starting with the core.abbrev length (usually 7)
and growing until it is no longer ambiguous.

pack-objects: free memory safely

A few fixes have accumulated in this area which have made the freeing of
data a bit muddy. Make sure to free the data only when needed and once.

When we are going to write a delta to the packfile, we need to free the
data, otherwise leave it. The current version of the code mixes up the
checks for po->data and po->delta_data.

Remove ignored files from the working directory if they were stashed

Added a test, that fails for #2133

BUGFIX - Fetching twice from the same remote causes a segfault

Fix the description for `GIT_FEATURE_SSH`.

caps: Rename to features to avoid confusion

Do not allow git_branch_create() to force update branch

Add project list and update readme

Add git_diff_buffers and git_patch_from_buffers

This adds `git_diff_buffers` and `git_patch_from_buffers`.  This
also includes a bunch of internal refactoring to increase the
shared code between these functions and the blob-to-blob and
blob-to-buffer APIs, as well as some higher level assert helpers
in the tests to also remove redundancy.

Blame first-parent history

Remove "ignore" state from the git.git-authors