Document how to build (and test) Git for Windows

[dscho]

This adds a section in the README.md file, documenting the advice I gave privately to a few colleagues over the past few days (which made me realize that this institutionalized knowledge lives in only one brain so far, and this PR changes that).

[jeffhostetler]

Historically, I use a different method that what you've listed here. (Guess
I should have asked about this years ago. 😄 )

I prepend the worktree root to my PATH or use ./git.exe to run the command.

Then I always add --exec-path=/c/<worktree>/ to my command lines. (Tedious,
yes, but I didn't know about GIT_EXEC_PATH until you mentioned it yesterday.)
However, this method is better for debugging in VS because I can just type the
exec-path arg along with the command line that I'm testing/debugging and not
worry about the environment that VS inherited.

Maybe add this usage as another choice?

[jeffhostetler]

One more case. When building core Git (instead of GFW), is there a preferred solution?

[jeffhostetler]

Hey, I forgot to say it earlier. Thanks for taking time to do this!!! I think it will be very helpful to help new folks get started.

[derrickstolee]

I prepend the worktree root to my PATH or use ./git.exe to run the command.

Then I always add --exec-path=/c/<worktree>/ to my command lines. (Tedious,
yes, but I didn't know about GIT_EXEC_PATH until you mentioned it yesterday.)
However, this method is better for debugging in VS because I can just type the
exec-path arg along with the command line that I'm testing/debugging and not
worry about the environment that VS inherited.

This has definitely been my approach before. It is insufficient for testing with the Scalar functional tests (unless I go edit the functional tests to point to that specific version of git.exe and also add the --exec-path to the C# code that calls Git). Other third-party tools shelling to Git will be harder to modify.

[dscho]

Historically, I use a different method that what you've listed here. (Guess
I should have asked about this years ago. 😄 )

:-)

I prepend the worktree root to my PATH or use ./git.exe to run the command.

How does this even work, given that this is missing all the .dll files required by git.exe to run? I guess you hard-linked (or copied) the respective .dll files?

When building core Git (instead of GFW), is there a preferred solution?

The easiest is what I would suggest as preferred solution, and I tried to order the options from easy to hard (in my very own judgemental way).

[dscho]

@jeffhostetler @derrickstolee I force-pushed. Could you give this another look-over?

[jeffhostetler]

This looks good! Thanks for doing this!