Add a failing route spec for file named diff.
Do not check for format on blob diff path.
The 'shell commands' guide also covers files/paths
Merge branch 'contributing-md-file-read' into 'master'

Add an extra reference to the 'shell commands' guide

The 'shell commands' guide now also has tips on opening/reading files and handling paths.

See merge request !1321
Merge branch 'show_files_with_diff_name' into 'master'

Show files named diff

Fixes #1749

See merge request !1318