log: fix indentation for --graph --show-signature

The git log --graph --show-signature command incorrectly indents the gpg
information about signed commits and merged signed tags. It does not
follow the level of indentation of the current commit.

Example of garbled output:
$ git log --show-signature --graph
*   commit 258e0a237cb69aaa587b0a4fb528bb0316b1b776
|\  gpg: Signature made Mon, Jun 30, 2014 13:22:33 EDT using RSA key ID DA08
gpg: Good signature from "Jason Pyeron <jpye...@pdinc.us>"
Merge: 727c355 1ca13ed
| | Author: Jason Pyeron <jpye...@pdinc.us>
| | Date:   Mon Jun 30 13:22:29 2014 -0400
| |
| |     Merge of 1ca13ed2271d60ba9 branch - rebranding
| |
| * commit 1ca13ed2271d60ba93d40bcc8db17ced8545f172
| | gpg: Signature made Mon, Jun 23, 2014  9:45:47 EDT using RSA key ID DD37
gpg: Good signature from "Stephen Robert Guglielmo <s...@guglielmo.us>"
gpg:                 aka "Stephen Robert Guglielmo <srguglie...@gmail.com>"
Author: Stephen R Guglielmo <s...@guglielmo.us>
| | Date:   Mon Jun 23 09:45:27 2014 -0400
| |
| |     Minor URL updates

In log-tree.c modify show_sig_lines() function to call graph_show_oneline()
after each line of gpg information it has printed in order to preserve
the level of indentation for the next output line.

Reported-by: Jason Pyeron <jpyeron@pdinc.us>
Signed-off-by: Zoltan Klinger <zoltan.klinger@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>