Fix various things that sparse complains about:
- use NULL instead of 0
- make sure we declare everything properly, or mark it static
- use proper function declarations ("fn(void)" instead of "fn()")
Sparse is always right.
This allows the programs to use various simplified versions of
the SHA1 names, eg just say "HEAD" for the SHA1 pointed to by
the .git/HEAD file etc.
For example, this commit has been done with
git-commit-tree $(git-write-tree) -p HEAD
instead of the traditional "$(cat .git/HEAD)" syntax.
This also regularizes the make. The source files themselves don't get
the "git-" prefix, because that's just inconvenient. So instead we just
make the rule that "git-xxxx" depends on "xxxx.c", and do that for
all the core programs (ie the old "git-mktag.c" got renamed to just
"mktag.c" to match everything else).
And "show-diff" got renamed to "git-diff-files" while at it, since
that's what it really should be to match the other git-diff-xxx cases.
