Fetch disassembly broken for source files that are symlinks. (sends link-target to gdb)

I'm on x86-64 Arch Linux (rolling release, updated last week), using python-gdbgui 0.7.9.5-1 built from https://aur.archlinux.org/packages/python-gdbgui/.

I have gdb 8.0 (from the Arch binary package), and my browser is Chromium 60.0.3112.101 (Developer Build) (64-bit) from the Arch binary package.

----

Disassembly output works with a hello world compiled with gcc, clang, g++, or clang++.

Disassembly output fails when the source file is a symlink. For example:

```
ln -s hello.c hello-link.c
gcc -g hello-link.c -o hello-link
```

Clicking "fetch disassembly" (even before the running the program) produces an error message in the gdb console:
`-data-disassemble: Invalid filename.` The message is also displayed in upper bar next to the a red "error" tag.

This happens with gcc7.1.1 / clang4.0.1 / g++ and clang++.

I used `strace -s1024 -p $(pidof gdb)` to see what command gdb actually received.

```
$ pgrep ...
/usr/bin/python /usr/bin/gdbgui ./hello-link
$ strace -s1024 -p $(pidof gdb)
strace: Process 16662 attached
restart_syscall(<... resuming interrupted restart_syscall ...>) = 1
read(0, "-", 1) = 1
read(0, "d", 1) = 1
read(0, "a", 1) = 1
read(0, "t", 1) = 1
read(0, "a", 1) = 1
read(0, "-", 1) = 1
read(0, "d", 1) = 1
read(0, "i", 1) = 1
read(0, "s", 1) = 1
read(0, "a", 1) = 1
read(0, "s", 1) = 1
read(0, "s", 1) = 1
read(0, "e", 1) = 1
read(0, "m", 1) = 1
read(0, "b", 1) = 1
read(0, "l", 1) = 1
read(0, "e", 1) = 1
read(0, " ", 1) = 1
read(0, "-", 1) = 1
read(0, "f", 1) = 1
read(0, " ", 1) = 1
read(0, "/", 1) = 1
read(0, "h", 1) = 1
read(0, "o", 1) = 1
read(0, "m", 1) = 1
read(0, "e", 1) = 1
read(0, "/", 1) = 1
read(0, "p", 1) = 1
read(0, "e", 1) = 1
read(0, "t", 1) = 1
read(0, "e", 1) = 1
read(0, "r", 1) = 1
read(0, "/", 1) = 1
read(0, "s", 1) = 1
read(0, "r", 1) = 1
read(0, "c", 1) = 1
read(0, "/", 1) = 1
read(0, "e", 1) = 1
read(0, "x", 1) = 1
read(0, "p", 1) = 1
read(0, "e", 1) = 1
read(0, "r", 1) = 1
read(0, "i", 1) = 1
read(0, "m", 1) = 1
read(0, "e", 1) = 1
read(0, "n", 1) = 1
read(0, "t", 1) = 1
read(0, "s", 1) = 1
read(0, "/", 1) = 1
read(0, "h", 1) = 1
read(0, "e", 1) = 1
read(0, "l", 1) = 1
read(0, "l", 1) = 1
read(0, "o", 1) = 1
read(0, ".", 1) = 1
read(0, "c", 1) = 1
read(0, " ", 1) = 1
read(0, "-", 1) = 1
read(0, "l", 1) = 1
read(0, " ", 1) = 1
read(0, "3", 1) = 1
read(0, " ", 1) = 1
read(0, "-", 1) = 1
read(0, "n", 1) = 1
read(0, " ", 1) = 1
read(0, "1", 1) = 1
read(0, "0", 1) = 1
read(0, "0", 1) = 1
read(0, " ", 1) = 1
read(0, "-", 1) = 1
read(0, "-", 1) = 1
read(0, " ", 1) = 1
read(0, "4", 1) = 1
read(0, "\n", 1) = 1
lstat("/home", {st_mode=S_IFDIR|0755, st_size=10, ...}) = 0
lstat("/home/peter", {st_mode=S_IFDIR|0755, st_size=1636, ...}) = 0
lstat("/home/peter/src", {st_mode=S_IFDIR|0755, st_size=214, ...}) = 0
lstat("/home/peter/src/experiments", {st_mode=S_IFDIR|0755, st_size=3284, ...}) = 0
lstat("/home/peter/src/experiments/hello.c", {st_mode=S_IFREG|0644, st_size=64, ...}) = 0
write(1, "^error,msg=\"-data-disassemble: Invalid filename.\"\n(gdb) \n", 57) = 57
read(7, 0x7ffc9861fcd7, 1) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=3, events=POLLIN}, {fd=7, events=POLLIN}, {fd=0, events=POLLIN}], 3, 0) = 0 (Timeout)
poll([{fd=3, events=POLLIN}, {fd=7, events=POLLIN}, {fd=0, events=POLLIN}], 3, -1
^Cstrace: Process 16662 detached

```

**So gdbgui sent the symlink-target filename, not the original source filename given to the compiler.**

The bar next to the "fetch disassembly" button says /home/peter/src/experiments/hello.c, but `strings -a hello-link` shows that `hello-link.c` appears in the file, but `hello.c` doesn't.

So gdb doesn't think hello.c is involved at all, but that's the file gdbgui is asking it for disassembly of.