This patch hails from Git for Windows (where the Cygwin runtime is used
in the form of a slightly modified MSYS2 runtime), where it is a
well-established technique to let the `$HOME` variable define where the
current user's home directory is, falling back to `$HOMEDRIVE$HOMEPATH`
and `$USERPROFILE`.
The idea is that we want to share user-specific settings between
programs, whether they be Cygwin, MSYS2 or not. Unfortunately, we
cannot blindly activate the "db_home: windows" setting because in some
setups, the user's home directory is set to a hidden directory via an
UNC path (\\share\some\hidden\folder$) -- something many programs
cannot handle correctly, e.g. `cmd.exe` and other native Windows
applications that users want to employ as Git helpers.
The established technique is to allow setting the user's home directory
via the environment variables mentioned above: `$HOMEDRIVE$HOMEPATH` or
`$USERPROFILE`. This has the additional advantage that it is much
faster than querying the Windows user database.
Of course this scheme needs to be opt-in. For that reason, it needs
to be activated explicitly via `db_home: env` in `/etc/nsswitch.conf`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
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