strbuf_expand: convert "%%" to "%"

The only way to safely quote arbitrary text in a pretty-print user
format is to replace instances of "%" with "%x25". This is slightly
unreadable, and many users would expect "%%" to produce a single
"%", as that is what printf format specifiers do.

This patch converts "%%" to "%" for all users of strbuf_expand():

 (1) git-daemon interpolated paths

 (2) pretty-print user formats

 (3) merge driver command lines

Case (1) was already doing the conversion itself outside of
strbuf_expand(). Case (2) is the intended beneficiary of this patch.
Case (3) users probably won't notice, but as this is user-facing
behavior, consistently providing the quoting mechanism makes sense.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -134,6 +134,7 @@ The placeholders are:
 - '%C(...)': color specification, as described in color.branch.* config option
 - '%m': left, right or boundary mark
 - '%n': newline
+- '%%': a raw '%'
 - '%x00': print a byte from a hex code
 - '%w([<w>[,<i1>[,<i2>]]])': switch line wrapping, like the -w option of
   linkgit:git-shortlog[1].

Documentation/technical/api-strbuf.txt

@@ -199,6 +199,10 @@ character if the letter `n` appears after a `%`.  The function returns
 the length of the placeholder recognized and `strbuf_expand()` skips
 over it.
 +
+The format `%%` is automatically expanded to a single `%` as a quoting
+mechanism; callers do not need to handle the `%` placeholder themselves,
+and the callback function will not be invoked for this placeholder.
++
 All other characters (non-percent and not skipped ones) are copied
 verbatim to the strbuf.  If the callback returned zero, meaning that the
 placeholder is unknown, then the percent sign is copied, too.

daemon.c

@@ -147,7 +147,6 @@ static char *path_ok(char *directory)
 			{ "IP", ip_address },
 			{ "P", tcp_port },
 			{ "D", directory },
-			{ "%", "%" },
 			{ NULL }
 		};
 

strbuf.c

@@ -227,6 +227,12 @@ void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn,
 			break;
 		format = percent + 1;
 
+		if (*format == '%') {
+			strbuf_addch(sb, '%');
+			format++;
+			continue;
+		}
+
 		consumed = fn(sb, format, context);
 		if (consumed)
 			format += consumed;

t/t6006-rev-list-format.sh

@@ -19,6 +19,13 @@ test_cmp expect.$1 output.$1
 "
 }
 
+test_format percent %%h <<'EOF'
+commit 131a310eb913d107dd3c09a65d1651175898735d
+%h
+commit 86c75cfd708a0e5868dc876ed5b8bb66c80b4873
+%h
+EOF
+
 test_format hash %H%n%h <<'EOF'
 commit 131a310eb913d107dd3c09a65d1651175898735d
 131a310eb913d107dd3c09a65d1651175898735d