Generalize the handling of paths and BSD tar

[pkkm]

This is my proposed minor redesign of elpa-mirror with the goal to:

• Make it more predictable and easier to use for people who have tar or cygpath in an unusual place.
• Make it usable with any combination of operating system and tar variant, for example bsdtar on Linux.
• Improve performance.

The changes can be summarized as:

• Replace Windows executable finding logic with two defcustoms, elpamr-tar-executable and elpamr-cygpath-executable. This will force Windows users who have tar and cygpath in C:\cygwin64\bin or a similar location to add two lines to their init.el that they didn't need before. However, I still think this is overall the better solution, for two reasons. First, it lets elpa-mirror be used by people who have tar in an unusual location like C:\Program Files (x86)\MSYS2\usr\bin or /home/pkkm/.bin. Second, that's what users probably expect from other packages—for example, Magit uses a magit-git-executable variable.
• Don't use the shell unnecessarily.
• Check separately for macOS and BSD tar when each is relevant.
• Check for BSD tar only once per run of elpamr-create-mirror-for-installed.

Most of the changes also happen to improve performance. Here is the time it takes to recreate my repo at various commits, average of 100 runs:

Commit	Time (s)
master	1.74
Remove unnecessary shell usage in elpamr--is-mac	1.09
Separate the check for BSD tar and the check for macOS	0.90
Check for BSD tar only once per run	0.62

So this PR more than doubles the performance of elpa-mirror on my machine.

[redguardtoo]

Ok, could you rebase on master branch and squash the commit into one commit?

[pkkm]

OK!

[redguardtoo]

thanks

[redguardtoo]

Windows 10 users don't need set up tar location since tar is already part of system, right? I don't have Windows 10 right now.

[pkkm]

I don't have it either, but this official article says that Windows ships with BSD tar starting with an Insider Build from December 2017. So yes, I think that everyone who uses Windows 10 from 2018 or later should have it.

[pkkm]

Thanks for creating this useful package, by the way!