Correct new compiler warnings in builtin-revert

The new builtin-revert code introduces a few new compiler errors
when I'm building with my stricter set of checks enabled in CFLAGS.
These all just stem from trying to store a constant string into
a non-const char*.  Simple fix, make the variables const char*.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

builtin-revert.c

@@ -235,8 +235,8 @@ static int revert_or_cherry_pick(int argc, const char **argv)
 	unsigned char head[20];
 	struct commit *base, *next;
 	int i;
-	char *oneline, *encoding, *reencoded_message = NULL;
-	const char *message;
+	char *oneline, *reencoded_message = NULL;
+	const char *message, *encoding;
 
 	git_config(git_default_config);
 	me = action == REVERT ? "revert" : "cherry-pick";

cache.h

@@ -449,7 +449,7 @@ extern int check_repository_format_version(const char *var, const char *value);
 extern char git_default_email[MAX_GITNAME];
 extern char git_default_name[MAX_GITNAME];
 
-extern char *git_commit_encoding;
+extern const char *git_commit_encoding;
 extern const char *git_log_output_encoding;
 
 extern int copy_fd(int ifd, int ofd);

environment.c

@@ -20,7 +20,7 @@ int is_bare_repository_cfg = -1; /* unspecified */
 int log_all_ref_updates = -1; /* unspecified */
 int warn_ambiguous_refs = 1;
 int repository_format_version;
-char *git_commit_encoding;
+const char *git_commit_encoding;
 const char *git_log_output_encoding;
 int shared_repository = PERM_UMASK;
 const char *apply_default_whitespace;