debian: install dashed commands to /usr/lib again
Various packages such as git-absorb, git-flow, and git-restore-mtime
install commands to /usr/lib/git-core and expect Git to be able to use
them as subcommands. Thus switching back to the default gitexecdir
(/usr/libexec/git-core) breaks those packages. Arguably they should
use /usr/bin instead, but this point in the Debian release cycle is
not the right time to make such a change.
This effectively reverts commit
57ed86e44b476aa258172c06429487c0583e01ba.
Bug: https://bugs.debian.org/985416
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>