RFR: 8251549: Update docs on building for Git

[Magnus Ihse Bursie]

On Mon, 26 Oct 2020 12:50:59 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> @jddarcy Recommended new wording:
>> Suggestion:
>> 
>>       * 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).
>
> I would recommend putting in a link to the instructions on the Skara wiki page. They actually recommend Git for Windows and I have switched too. The main reason is to be able to use the token manager. To just clone and build, using Cygwin Git is definitely easier though.

@erikj79 

Ok, here is a completely rewritten portion, which highlights the differences between Cygwin git and Git for Windows.

Suggestion:

      * You need to install a git client. You have two choices, Cygwin git or
        Git for Windows. Unfortunately there are pros and cons with each choice.

        * The Cygwin `git` client has no line ending issues and understands
          Cygwin paths (which are used throughout the JDK build system).
          However, it does not currently work well with the Skara CLI tooling.
          Please see the [Skara wiki on Git clients](
          https://wiki.openjdk.java.net/display/SKARA/Skara#Skara-Git) for
          up-to-date information about the Skara git client support.

        * The [Git for Windows](https://gitforwindows.org) client has issues
          with line endings, and do not understand Cygwin paths. It does work
          well with the Skara CLI tooling, however. To alleviate the line ending
          problems, make sure you set `core.autocrlf` to `false` (this is asked
          during installation).

@jddarcy Do you find this acceptable? This PR has been open for almost a month, and it's starting getting embarrassing that our build instruction does not match the new Github reality. :(

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