General const correctness fixes

We shouldn't attempt to assign constant strings into char*, as the
string is not writable at runtime.  Likewise we should always be
treating unsigned values as unsigned values, not as signed values.

Most of these are very straightforward.  The only exception is the
(unnecessary) xstrdup/free in builtin-branch.c for the detached
head case.  Since this is a user-level interactive type program
and that particular code path is executed no more than once, I feel
that the extra xstrdup call is well worth the easy elimination of
this warning.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-blame.c

@@ -1244,26 +1244,26 @@ static void pass_blame(struct scoreboard *sb, struct origin *origin, int opt)
  */
 struct commit_info
 {
-	char *author;
-	char *author_mail;
+	const char *author;
+	const char *author_mail;
 	unsigned long author_time;
-	char *author_tz;
+	const char *author_tz;
 
 	/* filled only when asked for details */
-	char *committer;
-	char *committer_mail;
+	const char *committer;
+	const char *committer_mail;
 	unsigned long committer_time;
-	char *committer_tz;
+	const char *committer_tz;
 
-	char *summary;
+	const char *summary;
 };
 
 /*
  * Parse author/committer line in the commit object buffer
  */
 static void get_ac_line(const char *inbuf, const char *what,
-			int bufsz, char *person, char **mail,
-			unsigned long *time, char **tz)
+			int bufsz, char *person, const char **mail,
+			unsigned long *time, const char **tz)
 {
 	int len;
 	char *tmp, *endp;
@@ -1280,7 +1280,7 @@ static void get_ac_line(const char *inbuf, const char *what,
 	if (bufsz <= len) {
 	error_out:
 		/* Ugh */
-		person = *mail = *tz = "(unknown)";
+		*mail = *tz = "(unknown)";
 		*time = 0;
 		return;
 	}