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.