RFR: 8251549: Update docs on building for Git

[Sergey Bylokhov]

On Mon, 7 Sep 2020 07:20:02 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> 8251549: Update docs on building for Git
>
> doc/building.md line 92:
> 
>> 90:         spaces and/or mixed upper and lower case letters.
>> 91:
>> 92:       * Clone the JDK repository using [Git for Windows](https://gitforwindows.org).
> 
> I think I said before, this is *not* a recommended way! Instead, git from Cygwin is still recommended.
> 
> Git for Windows will for instance set the coreautolf flag. Now we have pushed a work-around for this, but I object
> to *recommending* git for windows.

BTW the value of coreautolf flag is asked during the installation of Git for windows, it is should be mentioned in the
docs, since it is quite common issue.

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