sha1dc: build git plumbing code more explicitly

sha1dc: build git plumbing code more explicitly

The plumbing code between sha1dc and git is defined in
sha1dc_git.[ch], but these aren't compiled / included directly but
only via the indirect inclusion from sha1dc code.  This is slightly
confusing when you try to trace the build flow.

This patch brings the following changes for simplification:

  - Make sha1dc_git.c stand-alone and build from Makefile

  - sha1dc_git.h is the common header to include further sha1.h
    depending on the build condition

  - Move comments for plumbing codes from the header to definitions

This is also meant as a preliminary work for further plumbing with
external sha1dc shlib.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>