system_path(): Add prefix computation at runtime if RUNTIME_PREFIX set

This commit modifies system_path() to compute the prefix at runtime if
configured to do so.  If RUNTIME_PREFIX is defined, system_path() tries
to strip known directories that executables can be located in from the
path of the executable.  If the path is successfully stripped it is used
as the prefix.  For example, if the executable is "/msysgit/bin/git" and
BINDIR is "/bin", then the prefix is computed as "/msysgit".

We report an error if the runtime prefix computation fails, which can
happen if the executable is not installed at a known location.  The user
should know that the global configuration is not picked up, because this
can cause unexpected behavior.  If we explicitly want to ignore system
wide paths, we can set the environment variable GIT_CONFIG_NOSYSTEM, as
our tests do.

The implementation requires that argv0_path is set up properly, which is
currently the case only on Windows.  argv0_path must point to the
absolute path of the directory of the executable, which is verified by
two calls to assert().  On Windows, the wrapper for main() (see
compat/mingw.h) guarantees that this is the case.  On Unix, further work
is required before RUNTIME_PREFIX can be enabled.

Makefile

@@ -989,6 +989,9 @@ ifdef INTERNAL_QSORT
 	COMPAT_CFLAGS += -DINTERNAL_QSORT
 	COMPAT_OBJS += compat/qsort.o
 endif
+ifdef RUNTIME_PREFIX
+	COMPAT_CFLAGS += -DRUNTIME_PREFIX
+endif
 
 ifdef THREADED_DELTA_SEARCH
 	BASIC_CFLAGS += -DTHREADED_DELTA_SEARCH

exec_cmd.c

@@ -9,11 +9,48 @@ static const char *argv0_path;
 
 const char *system_path(const char *path)
 {
-	if (!is_absolute_path(path) && argv0_path) {
-		struct strbuf d = STRBUF_INIT;
-		strbuf_addf(&d, "%s/%s", argv0_path, path);
-		path = strbuf_detach(&d, NULL);
+#ifdef RUNTIME_PREFIX
+	static const char *prefix;
+
+	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;
+				strbuf_add(&d, argv0_path, sargv - argv0_path);
+				prefix = strbuf_detach(&d, NULL);
+				break;
+			}
+		}
 	}
+
+	if (!prefix) {
+		fprintf(stderr, "RUNTIME_PREFIX requested for path '%s', "
+				"but prefix computation failed.\n", path);
+		return path;
+	}
+
+	struct strbuf d = STRBUF_INIT;
+	strbuf_addf(&d, "%s/%s", prefix, path);
+	path = strbuf_detach(&d, NULL);
+#endif
 	return path;
 }