RFR: Makefile

[Jesper Wilhelmsson]

The build breaks if there are .md files not tracked by git because `git log` returns no hash.

-------------

Commit messages:
 - Fixed linebreak
 - Handle new files

Changes: https://git.openjdk.java.net/guide/pull/20/files
 Webrev: https://webrevs.openjdk.java.net/guide/20/webrev.00
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/guide/pull/20.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/20/head:pull/20

PR: https://git.openjdk.java.net/guide/pull/20