There are two different static functions and one global function,
all of them called "merge_file()", with different signatures and
purposes.  Rename them all to reduce confusion in "git grep" output:

 * Rename the static one in merge-index to "merge_one_path(const char
   *path)" as that function is about asking an external command to
   resolve conflicts in one path.

 * Rename the global one in merge-file.c that is only used by
   merge-tree to "merge_blobs()", as the function takes three blobs and
   returns the merged result only in-core, without doing anything to
   the filesystem.

 * Rename the one in merge-recursive to "merge_one_file()", just to be
   fair.

Also rename merge-file.[ch] to merge-blobs.[ch].

Signed-off-by: Junio C Hamano <gitster@pobox.com>