plumbing: fix pack commands for the file client on Windows

[strib]

The default git install on Windows doesn't come with commands for receive-pack and upload-pack in the default $PATH. Instead, use --exec-path to find pack executables in that case.

[smola]

@strib Thanks for your contribution!

As far as I know, git does have git-receive-pack and git-upload-pack binaries in Windows, but most installations do not update the PATH to contain the git exec path. I would prefer this to be implemented without build conditionals with the following logic:

1. If command is in path (with LookPath), then use it.
2. Otherwise, execute git --exec-path, which prints the git exec path (e.g. /usr/lib/git-core on most Linux distros) and then use it.

This should work on a wider range of scenarios, not only Windows, but also weird Linux or macOS setups.

[strib]

Cool thanks, I'll look doing things that way as soon as I get a chance.

Are those appveyor failures my fault, or known failures?

[mcuadros]

@strib No the appveyor issues are being fixed #345

[codecov]

Codecov Report

Merging #527 into master will decrease coverage by 0.65%.
The diff coverage is 63.33%.

@@            Coverage Diff             @@
##           master     #527      +/-   ##
==========================================
- Coverage   78.17%   77.51%   -0.66%     
==========================================
  Files         129      129              
  Lines        9749     9777      +28     
==========================================
- Hits         7621     7579      -42     
- Misses       1302     1380      +78     
+ Partials      826      818       -8

Impacted Files	Coverage Δ
plumbing/transport/file/client.go	81.15% <63.33%> (-13.97%)	⬇️
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	33.33% <0%> (-24.77%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b29ccd9...c128f5d. Read the comment docs.

[strib]

@smola @mcuadros this is ready for review again. I even added a test, though I can't run it on Windows due to #345. (I don't think I caused the travis failure above either, but let me know if I'm wrong.)

Thanks!

[mcuadros]

Great job! Can you squash the commit? And then I will merge it. Thanks

[strib]

@mcuadros done, thanks!