diff --relative: help working in a bare repository

This allows the --relative option to say which subdirectory to
pretend to be in, so that in a bare repository, you can say:

    $ git log --relative=drivers/ v2.6.20..v2.6.22 -- drivers/scsi/

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -170,10 +170,13 @@ endif::git-format-patch[]
 	Swap two inputs; that is, show differences from index or
 	on-disk file to tree contents.
 
---relative::
+--relative[=<path>]::
 	When run from a subdirectory of the project, it can be
 	told to exclude changes outside the directory and show
-	pathnames relative to it with this option.
+	pathnames relative to it with this option.  When you are
+	not in a subdirectory (e.g. in a bare repository), you
+	can name which subdirectory to make the output relative
+	to by giving a <path> as an argument.
 
 --text::
 	Treat all files as text.

diff.c

@@ -2302,6 +2302,10 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->detect_rename = 0;
 	else if (!strcmp(arg, "--relative"))
 		DIFF_OPT_SET(options, RELATIVE_NAME);
+	else if (!prefixcmp(arg, "--relative=")) {
+		DIFF_OPT_SET(options, RELATIVE_NAME);
+		options->prefix = arg + 11;
+	}
 
 	/* xdiff options */
 	else if (!strcmp(arg, "-w") || !strcmp(arg, "--ignore-all-space"))

revision.c

@@ -720,7 +720,7 @@ void init_revisions(struct rev_info *revs, const char *prefix)
 	revs->commit_format = CMIT_FMT_DEFAULT;
 
 	diff_setup(&revs->diffopt);
-	if (prefix) {
+	if (prefix && !revs->diffopt.prefix) {
 		revs->diffopt.prefix = prefix;
 		revs->diffopt.prefix_length = strlen(prefix);
 	}