typo in history
Support {a: {$regex: '', $options: 'i'}}

Fixes #1874.
arguably support empty regexs in file watcher
minimongo: support semi-logical $elemMatch

ie, a document with some logical operator and some equality in an
elemMatch.

eg: {a: {$elemMatch: {x: 1, $or: [{a: 1}, {b: 1}]}}}

Fixes #1875.
history update
Make paths relative in .gitignore

Some tools like `ag` don't work well with absolute paths but work well with
relative ones. Considering that this project-wise '.gitignore' stays in root, it
will be OK to change the paths to relative.