A leading / in paths in a .gitignore file matches the beginning of the
path, meaning that for patterns without slashes, git will match files
only in the current directory as opposed to in any subdirectory.
Prefix relevant paths with / in .gitignore files, to prevent
accidentally ignoring files in subdirectories and possibly slightly
improve the performance of "git status".
This is not near as fancy as as the unit tests, on the theory that
the code should typically be crashing when performance tuning.
Nonetheless, there is plenty of room for improvement. Several more of
the pieces of the test infrastructure (e.g. the option parsing) could
be factored out into test/test-lib-common.sh
