use of IFS= breaks GfW

This is a formalization of troubleshooting made while investigating https://github.com/dspinellis/git-issue/pull/60.

The issue is with `IFS=` and windows' propensity to use `C:` at the start of paths.

For instance, consider this:

```bash
echo -e "executable is [$0]"
echo "dirname is [$(dirname $0)/../lib]"
```

If added to `git-issue.sh` before (< 33) or after (> 51) the block of code that overrides `IFS`, I see

```
executable is [C:/Users/r2/AppData/Roaming/git-issue/bin/git-issue]
dirname is [C:/Users/r2/AppData/Roaming/git-issue/bin/../lib]
```

but between lines 33 and 51 (around your `LIB_PATH` code where `IFS=":"` temporarily), I see

```
executable is [C:/Users/r2/AppData/Roaming/git-issue/bin/git-issue]
dirname0 is [.
/Users/r2/AppData/Roaming/git-issue/bin]
```

The easy looping of `LIB_PATH` by using `IFS` is broken by that executable.

(I thought having this as a separate issue would do better to track the problem, vice discussing too long within the PR.)