Fix inconsistent definition of off_t on Linux

In order to avoid inconsistent definitions of type off_t, all
compilation units should include the "common.h" header file
before certain system headers (those which directly or indirectly
lead to the definition of off_t). The "common.h" header contains
the definition of _FILE_OFFSET_BITS to select 64-bit file offsets.

The symptom of this inconsistency, while compiling with -Wextra, is
the following warning:

    In file included from src/common.h:50,
                     from src/commit.c:28:
    src/util.h: In function git__is_sizet:
    src/util.h:41: warning: comparison between signed and unsigned

In order to fix the problem, we simply remove the #include <time.h>
statement at the head of src/commit.c.  Note that src/commit.h also
includes <time.h>.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Andreas Ericsson <ae@op5.se>

src/commit.c

@@ -23,8 +23,6 @@
  * Boston, MA 02110-1301, USA.
  */
 
-#include <time.h>
-
 #include "common.h"
 #include "commit.h"
 #include "revwalk.h"