Add missing error check

* src/git2r_merge.c (git2r_normal_merge): Check result of creating the
  merge commit.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Remove duplicate commit code

* src/git2r_commit.c: Use the same internal function to create a
  normal commit and a merge commit.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Remove unused argument

* src/git2r_commit.c (git2r_commit): Remove unused argument
  parent_list.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>