Improve git-annex's ability to find the path to its program, especially
when it needs to run itself in another repo to upgrade it.

Some parts of the code used readProgramFile, probably because I forgot that
programPath exists.

I noticed this when a git-annex auto-upgrade failed because it was running
git-annex upgrade --autoonly, but the code to run git-annex used
readProgramFile, which happened to point to an older build of git-annex.