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.