Moved pip install to the start, needed by the rest of the script.

.githooks/post-merge

 #!/bin/sh
 set -eu
 cd "$(git rev-parse --show-toplevel)"
+echo Installing Python apps >> /dev/stderr
+python3 -m pip install --user --requirement requirements.txt
 echo Installing Git hooks >> /dev/stderr
 Documents/bin/install-git-hooks
 echo Generating SSH config >> /dev/stderr
@@ -13,5 +15,3 @@ echo Creating Bash completion scripts >> /dev/stderr
 Documents/bin/gen-bash-completion
 echo Adding Cron job >> /dev/stderr
 Documents/bin/cron-jobs
-echo Installing Python apps >> /dev/stderr
-python3 -m pip install --user --requirement requirements.txt