[rkennedy]

The sh.Exec family of functions used to use os.Expand to expand environment variables in command arguments, but that function offers no escape syntax — no way to avoid expanding dollar signs that the caller really wants to include on the command line. This commit implements a new, mostly compatible Expand function to allow escaping the environment-variable syntax, plus an accompanying Escape function to producing escaped strings.

This technically breaks backward compatibility; any existing code that has used the sequences "\\" or "\$" in its command arguments will need to be updated. Although the compatibility argument prevents os.Expand from changing, documentation for sh.Exec hasn't actually stated that it follows all the same rules as os.Expand, so it's possible to view this change as merely a bugfix, formalizing a new syntax that had not previously been established.

Fixes #434

[natefinch]

Thanks for tackling literal dollar signs in command arguments. I applied this patch to current master (0953947) to review it. I found two compatibility regressions that need addressing before merging; the inline comments include reproductions. The Escape round-trip worked in the cases I exercised, but replacing os.Expand also changes behavior for existing arguments unrelated to dollar escaping.

The updated patch also needs a small lint cleanup: use strings.ReplaceAll, fix the spelling of preceding, and use t.Setenv in the precedence test.