Add linting for syntax brought in by unresolved merge conflicts.

Summary:
Add linting capability for detecting files which contain
syntax introduced by unresolved merge conflicts. The detection is
file-type-agnostic (the only requirement is that the file is a text
file).

Test Plan:
Tested in three ways.

The first way is to add all three forms of syntax to a file to
indicate a merge conflict. HPHP will pick this up as a syntax error
before this linter reaches it.

The second way is to add the syntax in a comment. In that case, this
linter will show three warnings. For example:

    $ arc lint ArcanistMergeConflictLinter.php
    >>> Lint for arcanist/src/lint/linter/ArcanistMergeConflictLinter.php:

       Warning  (MERGECONFLICT1) Unresolved merge conflict
        This syntax indicates there is still an unresolved merge conflict.

                  20
                  21     foreach ($lines as $lineno => $line) {
                  22 /*
        >>>       23 >>>>>>>
                  24
                  25 =======

       Warning  (MERGECONFLICT1) Unresolved merge conflict
        This syntax indicates there is still an unresolved merge conflict.

                  22 /*
                  23 >>>>>>>
                  24
        >>>       25 =======
                  26
                  27 <<<<<<<

       Warning  (MERGECONFLICT1) Unresolved merge conflict
        This syntax indicates there is still an unresolved merge conflict.

                  24
                  25 =======
                  26
        >>>       27 <<<<<<<
                  28
                  29 */

The last test was to test on various different file types, including
JavaScript, PHP, an animated GIF, a PNG, and a Bash file to make sure
the file type detection worked. Each of the aforementioned tests
passed.

Reviewers: vrana, epriestley

Reviewed By: epriestley

CC: aran, epriestley, Korvin

Maniphest Tasks: T2547

Differential Revision: https://secure.phabricator.com/D4966