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.

merge-base.c

@@ -58,8 +58,8 @@ int main(int argc, char **argv)
 	unsigned char rev1key[20], rev2key[20];
 
 	if (argc != 3 ||
-	    get_sha1_hex(argv[1], rev1key) ||
-	    get_sha1_hex(argv[2], rev2key)) {
+	    get_sha1(argv[1], rev1key) ||
+	    get_sha1(argv[2], rev2key)) {
 		usage("merge-base <commit-id> <commit-id>");
 	}
 	rev1 = lookup_commit(rev1key);