merge: allow custom conflict marker size

Allow for a custom conflict marker size, allowing callers to override
the default size of the "<<<<<<<" and ">>>>>>>" markers in the
conflicted output file.

include/git2/merge.h

@@ -162,6 +162,8 @@ typedef enum {
 	GIT_MERGE_FILE_DIFF_MINIMAL = (1 << 7),
 } git_merge_file_flag_t;
 
+#define GIT_MERGE_CONFLICT_MARKER_SIZE	7
+
 /**
  * Options for merging a file
  */
@@ -191,6 +193,10 @@ typedef struct {
 
 	/** see `git_merge_file_flag_t` above */
 	git_merge_file_flag_t flags;
+
+	/** The size of conflict markers (eg, "<<<<<<<").  Default is
+	 * GIT_MERGE_CONFLICT_MARKER_SIZE. */
+	unsigned short marker_size;
 } git_merge_file_options;
 
 #define GIT_MERGE_FILE_OPTIONS_VERSION 1

src/merge_file.c

@@ -126,6 +126,8 @@ static int merge_file__xdiff(
 	if (options.flags & GIT_MERGE_FILE_DIFF_MINIMAL)
 		xmparam.xpp.flags |= XDF_NEED_MINIMAL;
 
+	xmparam.marker_size = options.marker_size;
+
 	if ((xdl_result = xdl_merge(&ancestor_mmfile, &our_mmfile,
 		&their_mmfile, &xmparam, &mmbuffer)) < 0) {
 		giterr_set(GITERR_MERGE, "failed to merge files");