preserve shellcmdflag and normalize for /bin/sh

[mirza-s]

Fix shell related problems when shellcmdflag contains flags not understood by /bin/sh by resetting it to a safe value and restoring it at the end.

[mirza-s]

Related #988

[bhcleek]

Can you give a concrete example of what this fixes for you?

[mirza-s]

On Ubuntu 18.04 if you set shellcmdflag=-lc it breaks all operations that use the shell such as gofmt.
So you can't even save *.go files because gofmt is called before each save.

Error detected while processing function <SNR>24_fmt_autosave[3]..go#fmt#Format[41]..go#fmt#run[5]..go#util#Exec[17]..<SNR>41_exec[7]..<SNR>41_system:
line   10:
E484: Can't open file /tmp/vcxetHD/1
Error detected while processing function <SNR>24_fmt_autosave:
line    3:
E171: Missing :endif
Press ENTER or type command to continue

This is because /bin/sh does not understand -lc (on Ubuntu sh is linked to dash)

$ sh -lc echo foo
foo: 66: foo: Syntax error: "(" unexpected

This does not happen on OSX because sh is actually bash which understands the provided shellcmdflag

So we reset shellcmdflag to -c which is understood by sh and restore it after invocation.

[bhcleek]

Given that vim-go does not set shellcmdflag at all, it sounds like this is better taken care of in your vimrc than in vim-go.

[mirza-s]

But it replaces the shell that uses shellcmdflag.

[mirza-s]

Why was 881a9cc needed at all?

[bhcleek]

lgtm

[bhcleek]

Thank you for the contribution.

[mirza-s]

Thank you.