Fix 'Not possible to change sign ...' issue

Sorry I put stupid modification on #172 thus I rearrange the commits and request this pull.

When user remove the first line of the file, vim-gitgutter tried to
replace the sign of the line 0 and the following error would caused.

    Error detected while processing function
    gitgutter#process_buffer..sign#update_sings..sign#upsert_new_gitgutter_signs:
    line    13:
    E885: Not possible to change sign GitGutterLineRemoved

While Vim does not have line 0, place the sign at line 1 instead of line
0. It should be ok while if there are any modification at line 1, the
sign will b overwritten.

The reproducing procedure is

1. Open the file which is handled with git
2. Remove the first line and save it (No exception)
3. Close the Vim.
4. Reopen the file, it will cause the exception

Thanks.