ignore: add existence check for ignore files

[sharkdp]

This PR suggests to add a simple .exists() check for .gitignore, .ignore, and other similar files before actually calling File::open(…) in GitIgnoreBuilder::add.

The reason is that a simple existence check via stat can be faster than actually trying to open the file (see https://stackoverflow.com/a/12774387/704831). As we typically expect the number of directories without ignore files to be much larger than the number of directories with ignore files, this leads to an overall speedup.

The performance gain is not huge for rg, but can be quite significant if more .gitignore-like files are added via add_custom_ignore_filename. The speedup is larger for folders with low files-per-directory ratios.

Benchmark results

rg --files in my home folder (200k results, 6.5 files per directory):

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./rg-master --files	396.4 ± 3.2	390.9	400.0	1.05
./rg-feature --files	376.0 ± 3.6	369.3	383.5	1.00

rg --files --hidden in my home folder (800k results, 5.4 files per directory)

Command	Mean [s]	Min [s]	Max [s]	Relative
./rg-master --files --hidden	1.575 ± 0.012	1.560	1.597	1.06
./rg-feature --files --hidden	1.479 ± 0.011	1.464	1.496	1.00

rg --files in the chromium-79.0.3915.2 source tree (300k results, 12.7 files per directory)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
~/rg-master --files	445.2 ± 5.3	435.6	453.0	1.04
~/rg-feature --files	428.9 ± 7.0	418.2	440.0	1.00

rg --files in the linux-5.3 source tree (65k results, 15.1 files per directory)

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
./rg-master --files	94.5 ± 1.9	89.8	98.5	1.02
./rg-feature --files	92.6 ± 2.7	88.4	98.7	1.00

Running a fd search in my home directory (fd uses more .gitignore-like files):

Command	Mean [s]	Min [s]	Max [s]	Relative
./fd-master -H foobar	1.136 ± 0.012	1.122	1.158	1.10
./fd-feature -H foobar	1.030 ± 0.006	1.022	1.041	1.00

[sharkdp]

(I am assuming that the CI failure is not related to my PR. If this is not the case, please let me know)

[BurntSushi]

I buy it, thanks!