C implementation of the 'git' program, take two.

C implementation of the 'git' program, take two.

This patch provides a C implementation of the 'git' program and
introduces support for putting the git-* commands in a directory
of their own. It also saves some time on executing those commands
in a tight loop and it prints the currently available git commands
in a nicely formatted list.

The location of the GIT_EXEC_PATH (name discussion's closed, thank gods)
can be obtained by running

	git --exec-path

which will hopefully give porcelainistas ample time to adapt their
heavy-duty loops to call the core programs directly and thus save
the extra fork() / execve() overhead, although that's not really
necessary any more.

The --exec-path value is prepended to $PATH, so the git-* programs
should Just Work without ever requiring any changes to how they call
other programs in the suite.

Some timing values for 10000 invocations of git-var >&/dev/null:
	git.sh: 24.194s
	git.c:   9.044s
	git-var: 7.377s

The git-<tab><tab> behaviour can, along with the someday-to-be-deprecated
git-<command> form of invocation, be indefinitely retained by adding
the following line to one's .bash_profile or equivalent:

	PATH=$PATH:$(git --exec-path)

Experimental libraries can be used by either setting the environment variable
GIT_EXEC_PATH, or by using

	git --exec-path=/some/experimental/exec-path

Relative paths are properly grok'ed as exec-path values.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>