[jdk13u-dev] RFR: 8251549: Update docs on building for Git

Wed Apr 7 15:27:58 UTC 2021

It is long overdue update for jdk13u. Patch of original JDK-8251549 applies clean. In addition, I'm putting in the same jdk13u commit also building.html generated from the updated version of building.md.

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Commit messages:
 - Backport 042734cc5b17302a8f2ecdf577511bd6d5ec5e22

Changes: https://git.openjdk.java.net/jdk13u-dev/pull/174/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk13u-dev&pr=174&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251549
  Stats: 108 lines in 2 files changed: 16 ins; 61 del; 31 mod
  Patch: https://git.openjdk.java.net/jdk13u-dev/pull/174.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk13u-dev pull/174/head:pull/174

PR: https://git.openjdk.java.net/jdk13u-dev/pull/174