system_path(): simplify using strip_path_suffix(), and add suffix "git"

At least for the author of this patch, the logic in system_path() was
too hard to understand.  Using the function strip_path_suffix() documents
the idea of the code better.

The real change is to add the suffix "git", so that a runtime prefix will
be computed correctly even when the executable was called in /git/ as is
the case in msysGit (Windows insists to search the current directory
before the PATH when looking for an executable).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

exec_cmd.c

@@ -23,35 +23,10 @@ const char *system_path(const char *path)
 	assert(argv0_path);
 	assert(is_absolute_path(argv0_path));
 
-	if (!prefix) {
-		const char *strip[] = {
-			GIT_EXEC_PATH,
-			BINDIR,
-			0
-		};
-		const char **s;
-
-		for (s = strip; *s; s++) {
-			const char *sargv = argv0_path + strlen(argv0_path);
-			const char *ss = *s + strlen(*s);
-			while (argv0_path < sargv && *s < ss
-				&& (*sargv == *ss ||
-				    (is_dir_sep(*sargv) && is_dir_sep(*ss)))) {
-				sargv--;
-				ss--;
-			}
-			if (*s == ss) {
-				struct strbuf d = STRBUF_INIT;
-				/* We also skip the trailing directory separator. */
-				assert(sargv - argv0_path - 1 >= 0);
-				strbuf_add(&d, argv0_path, sargv - argv0_path - 1);
-				prefix = strbuf_detach(&d, NULL);
-				break;
-			}
-		}
-	}
-
-	if (!prefix) {
+	if (!prefix &&
+	    !(prefix = strip_path_suffix(argv0_path, GIT_EXEC_PATH)) &&
+	    !(prefix = strip_path_suffix(argv0_path, BINDIR)) &&
+	    !(prefix = strip_path_suffix(argv0_path, "git"))) {
 		prefix = PREFIX;
 		fprintf(stderr, "RUNTIME_PREFIX requested, "
 				"but prefix computation failed.  "