Use libcrypto's SHA-1 implementation when linking to it

libcryto's SHA-1 implementation is measurably better than the one that
ships with the library. If we link to it for HTTPS support already,
use that implementation instead.

Testing on a ~600MB of the linux repository, this reduces indexing
time by 40% and removes the hashing from the top spot in the perf
output.

Add git_config_level support when handling config files

Added `struct git_config_entry`: a git_config_entry contains the key, the value, and the config file level from which a config element was found.
Added `git_config_open_level`: build a single-level focused config object from a multi-level one.

We are now storing `git_config_entry`s in the khash of the config_file

This can occur after a manual modification of a config file.

Save the autotag configuration for remotes

More orphaned head love

blob: do not create temp files in the current path

 - make sure temporary streamed blobs are created under the
.git/objects folder and not in the current path, whatever it is.

 - do not make the name of the temp file depend on the hintpath.

Changes to CMake for packaging

Fix env variable tests with new Win32 path rules

Rename INSTALL_INC and INSTALL_BIN to INCLUDE_INSTALL_DIR and
BIN_INSTALL_DIR, which are more commonly used. This is also
consistent with the variable for the library path which is
already LIB_INSTALL_DIR.

The new Win32 global path search was not working with the
environment variable tests.  But when I fixed the test, the new
codes use of getenv() was causing more failures (presumably because
of caching on Windows ???).  This fixes the global file lookup to
always go directly to the Win32 API in a predictable way.

Add error GIT_EORPHANEDHEAD

Update README because vmg is @vmg.

Enhance test coverage

Update clar and add reliable rename for Win32

revparse: properly handle refnames containing a @

Fix #994

remote: support fetch cancelation

Introduce git_remote_stop() which sets a variable that is checked by
the fetch process in a few key places. If this is variable is set, the
fetch is aborted.

Support TYPECHANGE records in status and adjust checkout accordingly

Fixed no-submodule speedup of new checkout code.  Fixed missing
final update to progress (which may go away, I realize).  Fixed
unused structure in header and incorrect comment.