merge-ort: resolve paths early when we have sufficient information

When there are no directories involved at a given path, and all three
sides have a file at that path, and two of the three sides of history
match, we can immediately resolve the merge of that path in
collect_merge_info() and do not need to wait until process_entries().

This is actually a very minor improvement: half the time when I run it,
I see an improvement; the other half a slowdown.  It seems to be in the
range of noise.  However, this idea serves as the beginning of some
bigger optimizations coming in the following patches.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

merge-ort.c

@@ -1023,6 +1023,43 @@ static int collect_merge_info_callback(int n,
 		return mask;
 	}
 
+	/*
+	 * If the sides match, and all three paths are present and are
+	 * files, then we can take either as the resolution.  We can't do
+	 * this with trees, because there may be rename sources from the
+	 * merge_base.
+	 */
+	if (sides_match && filemask == 0x07) {
+		/* use side1 (== side2) version as resolution */
+		setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
+				names, names+1, side1_null, 0,
+				filemask, dirmask, 1);
+		return mask;
+	}
+
+	/*
+	 * If side1 matches mbase and all three paths are present and are
+	 * files, then we can use side2 as the resolution.  We cannot
+	 * necessarily do so this for trees, because there may be rename
+	 * destinations within side2.
+	 */
+	if (side1_matches_mbase && filemask == 0x07) {
+		/* use side2 version as resolution */
+		setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
+				names, names+2, side2_null, 0,
+				filemask, dirmask, 1);
+		return mask;
+	}
+
+	/* Similar to above but swapping sides 1 and 2 */
+	if (side2_matches_mbase && filemask == 0x07) {
+		/* use side1 version as resolution */
+		setup_path_info(opt, &pi, dirname, info->pathlen, fullpath,
+				names, names+1, side1_null, 0,
+				filemask, dirmask, 1);
+		return mask;
+	}
+
 	/*
 	 * Gather additional information used in rename detection.
 	 */