The bash module always assigns a value to HISTFILE in the bashrc, even
when no value is explicitly set. This makes it impossible to tell bash
to use a different HISTFILE by setting the HISTFILE environment variable
HISTFILE=/tmp/bash_history bash
This changes the default value of programs.bash.historyFile to null, and
only writes the HISTFILE=... line to the bashrc if it is changed to
something else.
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