Merge branch 'jc/am-read-author-file' into maint

Extract a small helper out of the function that reads the authors
script file "git am" internally uses.
This by itself is not useful until a second caller appears in the
future for "rebase -i" helper.

* jc/am-read-author-file:
  am: refactor read_author_script()

diff --cc builtin/am.c
Simple merge