RFR: JDK-8302323 Add repeat methods to StringBuilder/StringBuffer [v3]

[Jim Laskey]

> Add the ability to repeatedly append char and CharSequence data to StringBuilder/StringBuffer.

Jim Laskey has updated the pull request incrementally with one additional commit since the last revision:

  Optimize for empty CharSequence

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12728/files
  - new: https://git.openjdk.org/jdk/pull/12728/files/530f5543..01ffb19b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12728&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12728&range=01-02

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12728.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12728/head:pull/12728

PR: https://git.openjdk.org/jdk/pull/12728