RFR: 8251549: Update docs on building for Git [v2]

[Joe Darcy]

> 8251549: Update docs on building for Git

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Update doc/building.md
  
  Co-authored-by: Magnus Ihse Bursie <mag at icus.se>

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/21/files
  - new: https://git.openjdk.java.net/jdk/pull/21/files/185b04d4..91d91125

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=21&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=21&range=00-01

  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/21.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/21/head:pull/21

PR: https://git.openjdk.java.net/jdk/pull/21