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merge API
=========
 
The merge API helps a program to reconcile two competing sets of
improvements to some files (e.g., unregistered changes from the work
tree versus changes involved in switching to a new branch), reporting
conflicts if found.  The library called through this API is
responsible for a few things.
 
 * determining which trees to merge (recursive ancestor consolidation);
 
 * lining up corresponding files in the trees to be merged (rename
   detection, subtree shifting), reporting edge cases like add/add
   and rename/rename conflicts to the user;
 
 * performing a three-way merge of corresponding files, taking
   path-specific merge drivers (specified in `.gitattributes`)
   into account.
 
Low-level (single file) merge
-----------------------------
 
`ll_merge`::
 
	Perform a three-way single-file merge in core.  This is
	a thin wrapper around `xdl_merge` that takes the path and
	any merge backend specified in `.gitattributes` or
	`.git/info/attributes` into account.  Returns 0 for a
	clean merge.
 
The caller:
 
1. allocates an mmbuffer_t variable for the result;
2. allocates and fills variables with the file's original content
   and two modified versions (using `read_mmfile`, for example);
3. calls ll_merge();
4. reads the output from result_buf.ptr and result_buf.size;
5. releases buffers when finished (free(ancestor.ptr); free(ours.ptr);
   free(theirs.ptr); free(result_buf.ptr);).
 
If the modifications do not merge cleanly, `ll_merge` will return a
nonzero value and `result_buf` will generally include a description of
the conflict bracketed by markers such as the traditional `<<<<<<<`
and `>>>>>>>`.
 
The `ancestor_label`, `our_label`, and `their_label` parameters are
used to label the different sides of a conflict if the merge driver
supports this.
 
The `flag` parameter is a bitfield:
 
 - The least significant bit indicates whether this is an internal
   merge to consolidate ancestors for a recursive merge.
 
 - The next two bits allow local conflicts to be automatically
   resolved in favor of one side or the other (as in 'git merge-file'
   `--ours`/`--theirs`/`--union` for 01, 10, and 11, respectively).
 
Everything else
---------------
 
Talk about <merge-recursive.h> and merge_file():
 
 - merge_trees() to merge with rename detection
 - merge_recursive() for ancestor consolidation
 - try_merge_command() for other strategies
 - conflict format
 - merge options
 
(Daniel, Miklos, Stephan, JC)