Locate File: allow to search './path/to/something.txt'

Summary:
Before this change, if you search a file using Locate File,
this was the only accepted syntax for a full-path:

path/to/something.txt

After this change, some relative/absolute Unix-like variants
are also accepted:

./path/to/something.txt
/path/to/something.txt

Similar prefixes can be frequent when you quickly copy-paste things
from shell commands like 'grep' or 'find' etc.

Ref T15508

Test Plan:
Visit a lovely repository like Phorge and use Locate File with these:

1. src/applications/maniphest/editor/ManiphestEditEngine.php
2. /src/applications/maniphest/editor/ManiphestEditEngine.php
3. ./src/applications/maniphest/editor/ManiphestEditEngine.php

After this change, also 2. and 3. return the expected file.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15508

Differential Revision: https://we.phorge.it/D25320