Provide a dirname() function when NO_LIBGEN_H=YesPlease

When there is no `libgen.h` to our disposal, we miss the `dirname()`
function.

So far, we only had one user of that function: credential-cache--daemon
(which was only compiled when Unix sockets are available, anyway). But
now we also have `builtin/am.c` as user, so we need it.

Since `dirname()` is a sibling of `basename()`, we simply put our very
own `gitdirname()` implementation next to `gitbasename()` and use it
if `NO_LIBGEN_H` has been set.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/basename.c

@@ -1,4 +1,5 @@
 #include "../git-compat-util.h"
+#include "../strbuf.h"
 
 /* Adapted from libiberty's basename.c.  */
 char *gitbasename (char *path)
@@ -25,3 +26,46 @@ char *gitbasename (char *path)
 	}
 	return (char *)base;
 }
+
+char *gitdirname(char *path)
+{
+	char *p = path, *after_slash = NULL, c;
+	int dos_drive_prefix;
+
+	if (!p)
+		return ".";
+
+	if ((dos_drive_prefix = skip_dos_drive_prefix(&p)) && !*p) {
+		static struct strbuf buf = STRBUF_INIT;
+
+dot:
+		strbuf_reset(&buf);
+		strbuf_addf(&buf, "%.*s.", dos_drive_prefix, path);
+		return buf.buf;
+	}
+
+	/*
+	 * POSIX.1-2001 says dirname("/") should return "/", and dirname("//")
+	 * should return "//", but dirname("///") should return "/" again.
+	 */
+	if (is_dir_sep(*p)) {
+		if (!p[1] || (is_dir_sep(p[1]) && !p[2]))
+			return path;
+		after_slash = ++p;
+	}
+	while ((c = *(p++)))
+		if (is_dir_sep(c)) {
+			char *tentative = p - 1;
+
+			/* POSIX.1-2001 says to ignore trailing slashes */
+			while (is_dir_sep(*p))
+				p++;
+			if (*p)
+				after_slash = tentative;
+		}
+
+	if (!after_slash)
+		goto dot;
+	*after_slash = '\0';
+	return path;
+}

git-compat-util.h

@@ -255,6 +255,8 @@ struct itimerval {
 #else
 #define basename gitbasename
 extern char *gitbasename(char *);
+#define dirname gitdirname
+extern char *gitdirname(char *);
 #endif
 
 #ifndef NO_ICONV