Further tweaks to buildinfo

Summary:
Old idea with changes since previous release tag
didn't work good enough. In most of the cases tag
was done in a branch hence not actually reachable
from the master branch.

Now change since release is gone, and date of
the latest commit is used instead.

The date is displayed in format YYYY-MM-DD HH:mm
in the splash.

New bpy.app fields:

- build_commit_timestamp is an unix timestamp of
  the commit blender was build from.
- build_commit_date is a date of that commit.
- build_commit_time is a time of that commit.

Reviewers: campbellbarton

Differential Revision: http://developer.blender.org/D5

release/scripts/modules/sys_info.py

@@ -68,10 +68,11 @@ def write_sysinfo(op):
     output.write("\nBlender:\n")
     output.write(lilies)
     if bpy.app.build_branch and bpy.app.build_branch != "Unknown":
-        output.write("version %s, branch %r, chage %r, hash %r, %r\n" %
+        output.write("version %s, branch %r, commit date %r %r, hash %r, %r\n" %
             (bpy.app.version_string,
              bpy.app.build_branch,
-             bpy.app.build_change,
+             bpy.app.build_commit_date,
+             bpy.app.build_commit_time,
              bpy.app.build_hash,
              bpy.app.build_type))
     else: