Resolve conflict
Resolve conflict
People can symlink it if they want in unsafe mode
Don’t repeat yourself
Rename constants
Fix unruby paradigm
Remove return
Remove obvious comment
Rename variable
Validate file name as soon as possible
Validate the entire markup as soon as possible
Change exception type
Change exception type in tests
Add note about installing Xcode on the Mac in the Installation docs.

Xcode command-line tools (clang and ruby headers) are required for installing some of Jekyll's dependencies which create native extensions.

Closes #1561.
Update history to reflect fixing of #1561
Merge pull request #1490 from penibelst/allow-include-read-override

Extract include tag read file in a method
Update history to reflect merge of #1490