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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 `LL_OPT_VIRTUAL_ANCESTOR` bit indicates whether this is an
internal merge to consolidate ancestors for a recursive merge.
- The `LL_OPT_FAVOR_MASK` bits allow local conflicts to be automatically
resolved in favor of one side or the other (as in 'git merge-file'
`--ours`/`--theirs`/`--union`).
They can be populated by `create_ll_flag`, whose argument can be
`XDL_MERGE_FAVOR_OURS`, `XDL_MERGE_FAVOR_THEIRS`, or
`XDL_MERGE_FAVOR_UNION`.
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)