bdiff: give slight preference to removing trailing lines

[This change could be folded into the previous changeset to minimize the repo
churn ...]

Similar to the previous change, introduce an exception to the general
preference for matches in the middle of bdiff ranges: If the best match on the
B side starts at the beginning of the bdiff range, don't aim for the
middle-most A side match but for the earliest.

New (later) matches on the A side will only be considered better if the
corresponding match on the B side *not* is at the beginning of the range.
Thus, if the best (middle-most) match on the B side turns out to be at the
beginning of the range, the earliest match on the A side will be used.

The bundle size for 4.0 (hg bundle --base null -r 4.0 x.hg) happens to go from
22807275 to 22808120 bytes - a 0.004% increase.

mercurial/bdiff.c

@@ -184,7 +184,7 b' static int longest_match(struct bdiff_li'
 				mj = j;
 				mk = k;
 			} else if (k == mk) {
-				if (i > mi && i <= half) {
+				if (i > mi && i <= half && j > b1) {
 					/* same match but closer to half */
 					mi = i;
 					mj = j;

tests/test-bdiff.py

@@ -88,7 +88,7 b' print("Diff 1 to 3 lines - preference fo'
 showdiff('a\n', 'a\n' * 3)
 print("Diff 1 to 5 lines - preference for appending:")
 showdiff('a\n', 'a\n' * 5)
-print("Diff 3 to 1 lines - preference for balanced recursion:")
+print("Diff 3 to 1 lines - preference for removing trailing lines:")
 showdiff('a\n' * 3, 'a\n')
-print("Diff 5 to 1 lines - preference for balanced recursion:")
+print("Diff 5 to 1 lines - preference for removing trailing lines:")
 showdiff('a\n' * 5, 'a\n')

tests/test-bdiff.py.out

@@ -68,17 +68,15 b' showdiff('
   'a\na\na\na\na\n'):
  'a\n'
  2 2 '' -> 'a\na\na\na\n'
-Diff 3 to 1 lines - preference for balanced recursion:
+Diff 3 to 1 lines - preference for removing trailing lines:
 showdiff(
   'a\na\na\n',
   'a\n'):
- 0 2 'a\n' -> ''
  'a\n'
- 4 6 'a\n' -> ''
-Diff 5 to 1 lines - preference for balanced recursion:
+ 2 6 'a\na\n' -> ''
+Diff 5 to 1 lines - preference for removing trailing lines:
 showdiff(
   'a\na\na\na\na\n',
   'a\n'):
- 0 4 'a\na\n' -> ''
  'a\n'
- 6 10 'a\na\n' -> ''
+ 2 10 'a\na\na\na\n' -> ''