filemerge: use 'util.ellipsis' to trim custom conflict markers correctly

Before this patch, filemerge slices byte sequence directly to trim
conflict markers, but this may cause:

  - splitting at intermediate multi-byte sequence

  - incorrect calculation of column width (length of byte sequence is
    different from columns in display in many cases)

This patch uses 'util.ellipsis' to trim custom conflict markers
correctly, even if multi-byte characters are used in them.

mercurial/filemerge.py

@@ -290,12 +290,8 b' def _formatconflictmarker(repo, ctx, tem'
     if mark:
         mark = mark.splitlines()[0] # split for safety
 
-    # The <<< marks add 8 to the length, and '...' adds three, so max
-    # length of the actual marker is 69.
-    maxlength = 80 - 8 - 3
-    if len(mark) > maxlength:
-        mark = mark[:maxlength] + '...'
-    return mark
+    # 8 for the prefix of conflict marker lines (e.g. '<<<<<<< ')
+    return util.ellipsis(mark, 80 - 8)
 
 _defaultconflictmarker = ('{node|short} ' +
     '{ifeq(tags, "tip", "", "{tags} ")}' +

tests/test-conflict.t

@@ -72,9 +72,42 b' Verify line splitting of custom conflict'
   something
   >>>>>>> other: test 1
 
+Verify line trimming of custom conflict marker using multi-byte characters
+
+  $ hg up -q --clean .
+  $ python <<EOF
+  > fp = open('logfile', 'w')
+  > fp.write('12345678901234567890123456789012345678901234567890' +
+  >          '1234567890') # there are 5 more columns for 80 columns
+  > 
+  > # 2 x 4 = 8 columns, but 3 x 4 = 12 bytes
+  > fp.write(u'\u3042\u3044\u3046\u3048'.encode('utf-8'))
+  > 
+  > fp.close()
+  > EOF
+  $ hg add logfile
+  $ hg --encoding utf-8 commit --logfile logfile
+
+  $ cat >> .hg/hgrc <<EOF
+  > [ui]
+  > mergemarkertemplate={desc|firstline}
+  > EOF
+
+  $ hg -q --encoding utf-8 merge 1
+  warning: conflicts during merge.
+  merging a incomplete! (edit conflicts, then use 'hg resolve --mark')
+  [1]
+
+  $ cat a
+  <<<<<<< local: 123456789012345678901234567890123456789012345678901234567890\xe3\x81\x82... (esc)
+  something else
+  =======
+  something
+  >>>>>>> other: branch1
+
 Verify basic conflict markers
 
-  $ hg up -q --clean .
+  $ hg up -q --clean 2
   $ printf "\n[ui]\nmergemarkers=basic\n" >> .hg/hgrc
 
   $ hg merge 1