Validates problem. Checks for missing files. Prompts to confirm submi…

[dmalan]

This version now assumes that we'll maintain a https://github.com/submit50/submit50 repo in which problem-specific metadata lives on problem-specific branches.

• Validates problem by making sure https://github.com/submit50/submit50/blob/$problem/exclude exists.
• Downloads a per-problem (case-insensitive) exclude (i.e., .gitignore) file, per https://github.com/cs50/submit50/blob/v1.1.0/opt/cs50/submit50/bin/submit50#L30.
  • One such file at https://github.com/submit50/submit50/blob/mario/exclude.
  • Format is identical to standard exclude and .gitignore files, but comments are used to provide hints as to which files a problem expects. E.g., #mario.c means "we expect mario.c to exist but you can still submit without," in case students are using submit50 to submit some temporary work.
• Summarizes for student what will be submitted and what won't, per https://github.com/cs50/submit50/blob/v1.1.0/opt/cs50/submit50/bin/submit50#L195.
• Prompts student to confirm proceeding, per https://github.com/cs50/submit50/blob/v1.1.0/opt/cs50/submit50/bin/submit50#L202.

To test:

cd mario
touch mario.c LUIGI.C Makefile mario luigi core foo.h foo.c Makefile.extra
./submit50 mario

CC @brianyu28 @rbowden91 @glennholloway @dlloyd09 @thctamm @kzidane

[glennholloway]

Before exiting at line 39, delete $EXCLUDE.

[glennholloway]

May as well put $file in double quotes on line 48, in case a file name containing a space ever shows up.

[dmalan]

Thanks, @glennholloway!