merge-recursive: rename merge_file_1() and merge_content()

merge-recursive: rename merge_file_1() and merge_content()

Summary:
  merge_file_1()  -> merge_mode_and_contents()
  merge_content() -> handle_content_merge()

merge_file_1() is a very unhelpful name.  Rename it to
merge_mode_and_contents() to reflect what it does.

merge_content() calls merge_mode_and_contents() to do the main part of
its work, but most of this function was about higher level stuff, e.g.
printing out conflict messages, updating skip_worktree bits, checking
for ability to avoid updating the working tree or for D/F conflicts
being in the way, etc.  Since there are several handle_*() functions for
similar levels of checking and handling in merge-recursive.c (e.g.
handle_change_delete(), handle_rename_rename_2to1()), let's rename this
function to handle_content_merge().

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>