prefetch: fix the hg executable location for background prefetch
The background prefetch uses the incorrect executable while running
the prefetch command due to which there are no logs for the background
prefetch. This commit fixes that by specifying the correct executable.
- Ran all the tests.
- Tested the output of util.hgexecutable() with system hg and chg, and verified that it returns the correct path. As expected, util.hgcmd() returns the corresponding path for hg and chg in this case.
Reviewers: #fbhgext, durham
Reviewed By: #fbhgext, durham
Differential Revision: https://phab.mercurial-scm.org/D1600