RFR: 7903830: apidiff: Review glyphs used to indicate same/difference

[Jonathan Gibbons]

Please review a medium small fix to change the glyphs used to visually indicate whether elements are the same, different, or in the case of a two-way comparison, added or removed.

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Commit messages:
 - CODETOOLS-7903830: apidiff: Review glyphs used to indicate same/difference

Changes: https://git.openjdk.org/apidiff/pull/17/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=17&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903830
  Stats: 59 lines in 3 files changed: 48 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/apidiff/pull/17.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/17/head:pull/17

PR: https://git.openjdk.org/apidiff/pull/17