revision: use C99 declaration of variable in for() loop

There are certain C99 features that might be nice to use in our code
base, but we've hesitated to do so in order to avoid breaking
compatibility with older compilers. But we don't actually know if
people are even using pre-C99 compilers these days.

One way to figure that out is to introduce a very small use of a
feature, and see if anybody complains, and we've done so to probe
the portability for a few features like "trailing comma in enum
declaration", "designated initializer for struct", and "designated
initializer for array".  A few years ago, we tried to use a handy

    for (int i = 0; i < n; i++)
	use(i);

to introduce a new variable valid only in the loop, but found that
some compilers we cared about didn't like it back then.  Two years
is a long-enough time, so let's try it again.

If this patch can survive a few releases without complaint, then we
can feel more confident that variable declaration in for() loop is
supported by the compilers our user base use.  And if we do get
complaints, then we'll have gained some data and we can easily
revert this patch.

Helped-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1198,6 +1198,7 @@ endif
 # Set CFLAGS, LDFLAGS and other *FLAGS variables. These might be
 # tweaked by config.* below as well as the command-line, both of
 # which'll override these defaults.
+# Older versions of GCC may require adding "-std=gnu99" at the end.
 CFLAGS = -g -O2 -Wall
 LDFLAGS =
 CC_LD_DYNPATH = -Wl,-rpath,

revision.c

@@ -43,10 +43,15 @@ static inline int want_ancestry(const struct rev_info *revs);
 
 void show_object_with_name(FILE *out, struct object *obj, const char *name)
 {
-	const char *p;
-
 	fprintf(out, "%s ", oid_to_hex(&obj->oid));
-	for (p = name; *p && *p != '\n'; p++)
+	/*
+	 * This "for (const char *p = ..." is made as a first step towards
+	 * making use of such declarations elsewhere in our codebase.  If
+	 * it causes compilation problems on your platform, please report
+	 * it to the Git mailing list at git@vger.kernel.org. In the meantime,
+	 * adding -std=gnu99 to CFLAGS may help if you are with older GCC.
+	 */
+	for (const char *p = name; *p && *p != '\n'; p++)
 		fputc(*p, out);
 	fputc('\n', out);
 }