RFR: 8251549: Update docs on building for Git

[Magnus Ihse Bursie]

On Mon, 7 Sep 2020 10:05:25 GMT, Sergey Bylokhov <serb at openjdk.org> wrote:

>> 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.

@jddarcy Recommended new wording:
      * Clone the JDK repository using the Cygwin command line `git` client
        as instructed in this document.

        Using the Cygwin `git` client is the recommended way since it
        guarantees that there will be no line ending issues and it works well
        with other Cygwin tools. It is however also possible to use [Git for
        Windows](https://gitforwindows.org). If you chose this path, make sure
        to set `core.autocrlf` to `false` (this is asked during installation).

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