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diff --git a/third_party/git/ll-merge.h b/third_party/git/ll-merge.h
new file mode 100644
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+++ b/third_party/git/ll-merge.h
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+/*
+ * Low level 3-way in-core file merge.
+ */
+
+#ifndef LL_MERGE_H
+#define LL_MERGE_H
+
+#include "xdiff/xdiff.h"
+
+/**
+ *
+ * Calling sequence:
+ * ----------------
+ *
+ * - Prepare a `struct ll_merge_options` to record options.
+ *   If you have no special requests, skip this and pass `NULL`
+ *   as the `opts` parameter to use the default options.
+ *
+ * - Allocate an mmbuffer_t variable for the result.
+ *
+ * - Allocate and fill variables with the file's original content
+ *   and two modified versions (using `read_mmfile`, for example).
+ *
+ * - Call `ll_merge()`.
+ *
+ * - Read the merged content from `result_buf.ptr` and `result_buf.size`.
+ *
+ * - Release buffers when finished.  A simple
+ *   `free(ancestor.ptr); free(ours.ptr); free(theirs.ptr);
+ *   free(result_buf.ptr);` will do.
+ *
+ * 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.
+ */
+
+
+struct index_state;
+
+/**
+ * This describes the set of options the calling program wants to affect
+ * the operation of a low-level (single file) merge.
+ */
+struct ll_merge_options {
+
+	/**
+	 * Behave as though this were part of a merge between common ancestors in
+	 * a recursive merge (merges of binary files may need to be handled
+	 * differently in such cases, for example). If a helper program is
+	 * specified by the `[merge "<driver>"] recursive` configuration, it will
+	 * be used.
+	 */
+	unsigned virtual_ancestor : 1;
+
+	/**
+	 * Resolve local conflicts automatically in favor of one side or the other
+	 * (as in 'git merge-file' `--ours`/`--theirs`/`--union`).  Can be `0`,
+	 * `XDL_MERGE_FAVOR_OURS`, `XDL_MERGE_FAVOR_THEIRS`,
+	 * or `XDL_MERGE_FAVOR_UNION`.
+	 */
+	unsigned variant : 2;
+
+	/**
+	 * Resmudge and clean the "base", "theirs" and "ours" files before merging.
+	 * Use this when the merge is likely to have overlapped with a change in
+	 * smudge/clean or end-of-line normalization rules.
+	 */
+	unsigned renormalize : 1;
+
+	/**
+	 * Increase the length of conflict markers so that nested conflicts
+	 * can be differentiated.
+	 */
+	unsigned extra_marker_size;
+
+	/* Extra xpparam_t flags as defined in xdiff/xdiff.h. */
+	long xdl_opts;
+};
+
+/**
+ * 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.
+ */
+int ll_merge(mmbuffer_t *result_buf,
+	     const char *path,
+	     mmfile_t *ancestor, const char *ancestor_label,
+	     mmfile_t *ours, const char *our_label,
+	     mmfile_t *theirs, const char *their_label,
+	     struct index_state *istate,
+	     const struct ll_merge_options *opts);
+
+int ll_merge_marker_size(struct index_state *istate, const char *path);
+void reset_merge_attributes(void);
+
+#endif