Given:
 * `file = "foo.a.b"`
 * `osuf = ".a.b"`  -- Note the initial dot.
 * `new_osuf = "c"`

Before (bad, the last character of the filename is dropped):
  `dropTail (length osuf + 1) file <.> new_osuf == "fo.c"`
After (good):
  `stripExtension osuf file <.> new_osuf` == "foo.c"

This regression was introduced in commit c489af73 (#5554). That commit
fixed a similar but different bug, and care has been taken to not
reintroduce it (using the the newly introduced
`System.Filepath.stripExtension`).

Given:
 * `file = "foo.a.b"`
 * `osuf = "a.b"`
 * `new_osuf = "c"`

Before c489af73 (bad, the full suffix should get replaced):
  `replaceExtension file new_osuf == "foo.a.c"`
After c489af73 (good):
  `dropTail (length osuf + 1) file <.> new_osuf == "foo.c"`
After this commit (still good):
  `stripExtension osuf file <.> new_osuf == "foo.c"`

Reviewed by: bgamari

Differential Revision: https://phabricator.haskell.org/D1692

GHC Trac Issues: #9760