Add "get_sha1()" helper function.

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.

rev-tree.c

@@ -97,7 +97,7 @@ int main(int argc, char **argv)
 			arg++;
 			basemask |= 1<<nr;
 		}
-		if (nr >= MAX_COMMITS || get_sha1_hex(arg, sha1[nr]))
+		if (nr >= MAX_COMMITS || get_sha1(arg, sha1[nr]))
 			usage("rev-tree [--edges] [--cache <cache-file>] <commit-id> [<commit-id>]");
 		process_commit(sha1[nr]);
 		nr++;