8251549: Update docs on building for Git

[yan-too]

jdk15u is in process of migrating to Git (jdk15u-dev is already there), so we would need a variant of this document in 15u, too.
This exact patch applies clean.
Yes, clean with some chunks shifted the uniform 42 lines. These lines, it seems to me, belong mostly to updates JDK-8256182 and JDK-8247591 and are irrelevant to this task.

I have plan for making building.html as a separate task -- a backport of JDK-8257224 if possible but maybe not.

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Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed

Issue

• JDK-8251549: Update docs on building for Git

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.java.net/jdk15u-dev pull/18/head:pull/18
$ git checkout pull/18

Update a local copy of the PR:
$ git checkout pull/18
$ git pull https://git.openjdk.java.net/jdk15u-dev pull/18/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 18

View PR using the GUI difftool:
$ git pr show -t 18

Using diff file

Download this PR as a diff file:
https://git.openjdk.java.net/jdk15u-dev/pull/18.diff

[bridgekeeper]

👋 Welcome back yan! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[openjdk]

@yan-too This change now passes all automated pre-integration checks.

After integration, the commit message for the final commit will be:

8251549: Update docs on building for Git

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

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[mlbridge]

Webrevs

• 00: Full (3430dad8)

[yan-too]

/integrate

[openjdk]

@yan-too Pushed as commit 6cf1c0d.

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